Uterine vascular hemodynamics are key to a healthy pregnancy. In a study of 30 participants, Jaffa et al. did not find a difference in the systolic/diastolic ratio (S/D) of the uterine arteries (UtA) in the supine position (assessed 5 minutes following adoption of the position) compared with the lateral position between 35–40 weeks gestation (Jaffa et al., 2001). Note that an unchanged S/D ratio does not mean that resistance is unchanged; rather, it means that the resistance in systole and diastole have both not changed or have both changed proportionally such that the relative amplitude of the systolic velocity compared to the diastolic velocity is unchanged. An earlier and much larger (n = 90) study, Ryo et al. found an elevated UtA resistance index (RI) in association with the degree of inferior vena cava (IVC) compression in the supine position (assessed 5 minutes following adoption of the position) (Ryo et al., 1996). The RI is a more direct reflection of the actual resistance of the distal placental capillary bed to forward blood flow. Park and Hikada’s results are concordant, demonstrating increased UtA resistance when supine compared to when in the left lateral posture (Park and Hidaka, 1991). Of note, the findings of Ryo et al. were observed at 24–27 weeks (Ryo et al., 1996), which indicate that the effects of supine posture on uterine hemodynamics occur at least by the late second trimester. In the third trimester, Jeffreys et al. observed that blood flow in the right UtA is reduced by 35% and 13% during supine rest and supine exercise, respectively, compared to its value in the left lateral posture, and this correlated directly with birthweight and placenta weight at birth (Jeffreys et al., 2006). Their results also corroborate both Jaffa et al. and Ryo et al.’s—they observed an unchanged S/D ratio and increased resistance when supine compared to left lateral posture (Ryo et al., 1996; Jaffa et al., 2001). Most recently, in the supine posture at 34–38 weeks, Couper et al. utilized functional MRI and observed 27.0% and 19.0% reductions in left and right internal iliac artery blood flow, respectively, in comparison to the lateral position (Couper et al., 2021). Note that each internal iliac artery gives rise to the ipsilateral UtA and are the main blood supply of the uterus and placenta.

“Orthostatic uterovascular syndrome” (OUS), first described by Schneider et al, (1984), describes a condition where, when standing motionless and unsupported in the third trimester, the maternal heart rate (HR) is observed to undergo cyclic accelerations. The physiology behind OUS is thought to be that the gravid uterus compresses the pelvic vessels when standing, reducing blood flow velocity in the femoral vein, which is followed by maternal HR acceleration (compensating for reduced cardiac return and preload) and a spontaneous uterine contraction, which is followed by increased blood flow velocity in the femoral vein and normalization of the maternal HR (Schneider and Deckardt, 1991; Schneider et al., 1993). The maternal HR normalizes within 14 s following the beginning of a uterine contraction, and this in-phase occurrence strengthens our current understanding of OUS physiology (Schneider et al., 1993). Furthermore, serum catecholamines are known to increase in response to physical stress, such as the upright posture, and are also known to decrease uterine blood flow (Katz et al., 1991), which may be an additional contributing mechanism to OUS. In order to be classified as OUS, the maternal HR must have a regular occurrence of accelerations (3 accelerations in 10 min) with an amplitude of at least 15 beats/min above the baseline. In one study of uncomplicated pregnancies between 36–41 weeks, OUS was observed to occur in 33 of 55 (60%) participants (Schneider et al., 1984), and this agrees with another study by the same authors a decade later in which prevalence of OUS was observed to be 5% at 24 weeks’ gestation and reached a peak of 71% at 38 weeks’ gestation (Schneider et al., 1993). Furthermore, as gestation advances, there is a three-fold increase in the frequency of uterine contractions when upright in OUS compared to those occurring in the left lateral posture. The frequency of upright uterine contractions in OUS may be reduced by 20% by leaning forward slightly, bending at the waist. Eighty-percent of these contractions are not felt by the patient, yet hemodynamic effects, primarily in the maternal HR, can be observed. Despite these cyclic fluctuations in maternal HR, maternal blood pressure (BP) remains relatively stable with no change in the diastolic BP and a 1.4% reduction in the systolic BP at the peak of the maternal HR acceleration (Schneider et al., 1993). However, roughly 25% of fetuses experience a reduction in fetal HR variability during OUS, suggesting a fetal effect due to maternal posture-related hemodynamic changes (Schneider et al., 1984).

Additionally, Park and Hidaka demonstrated a significant reduction in the frequency and intensity of uterine contractions in the left lateral posture compared to supine, and this correlated directly with maternal cardiac output (CO) (Park and Hidaka, 1991). They attribute this physiology to compression of the IVC by the gravid uterus when supine and resulting hemodynamic effects similar to that seen in OUS, namely, lower extremity venous congestion, resultant maternal HR acceleration, a spontaneous uterine contraction, and restoration of maternal cardiac preload and output, followed by normalization of maternal HR. This cyclic pattern of maternal HR reductions following uterine contractions was also observed in 11% of participants in a study of 119 low-risk, non-labouring, pregnancies at term (Ibrahim et al., 2015); however, the cyclic changes in maternal HR occurred in both supine and non-supine horizontally-oriented postures, being mainly associated with uterine activity, meant that no impact on the fetal HR pattern was observed. In term, non-laboring pregnancies, Ibrahim et al. found that the periodic maternal HR changes due to uterine activity are significantly more common (85% vs. 11%) when in the right lateral posture compared to the left (Ibrahim et al., 2015). It is important to note that this physiology is similar to that observed in supine hypotensive syndrome (SHS), which we discuss fully in our review of maternal posture-physiology interactions. As for clinical implications of posture-related uterine contractions, beyond left-uterine displacement or shifting to a non-supine posture being a universally recommended component of intrauterine resuscitation, we found one case report suggesting that in pregnancies that are sensitive to changes in intravascular volume (e.g., aortic stenosis), avoidance of the supine posture may attenuate the impact of uterine contractions (and hence autotransfusion of approximately 400 mL into maternal central circulation with each contraction) on maternal hemodynamics during labor (Baird and Arkoosh, 2012).

Overall, these reports suggest that compression of venous circulation by the gravid uterus, whether standing upright or in the supine posture, results in reduced maternal cardiac preload, which may precipitate uterine contractions in an effort to restore preload. The mechanism behind this is unelucidated and a potential area of future research.

Jaffa et al. (N = 30) did not find a difference in the S/D ratio of the umbilical arteries (UA) in the supine posture (assessed 5 minutes following adoption of the position) compared with the lateral posture between 35–40 weeks gestation (Jaffa et al., 2001). These findings are corroborated by Silva et al. (N = 20) who were unable to demonstrate differences in the UA pulsatility index (PI) and UA S/D between the left lateral and supine postures (assessed 5 minutes after assuming the lateral posture and at five and 10 minutes after assuming the supine posture) between 36–40 weeks (Silva et al., 2017). These studies were small but are in harmony with the findings of Ryo et al. (N = 90), who were unable to demonstrate an effect of maternal supine posture on UA flow parameters (assessed 5 minutes following adoption of the posture) at 24–27 weeks (Ryo et al., 1996).

Khatib et al., however, investigated pregnancies between 36–40 weeks (N = 23) and found that UA PI and UA S/D both increased (p < 0.001) after changing to the supine posture from the left lateral posture (assessed 15 minutes following adoption of each posture), reflecting increased umbilical-placental vascular resistance to forward flow when supine (Khatib et al., 2014). Nakai et al. (N = 23) had similar results in pregnancies after 34 weeks where they demonstrated that the UA S/D’s were significantly lower in the prone posture (p < 0.01) and left lateral posture (p = 0.055) compared to the supine posture (Nakai et al., 1998). Similarly, in a study in pregnancies between 23–36 weeks gestation, van Katwijk and Wladimiroff (N = 27) found that the UA PI increased significantly upon maternal change from standing to the supine posture and decreased significantly upon maternal change from the supine posture to standing (assessed 2–4 min following adoption of the posture), a phenomenon that the authors attributed to the “sluice flow” effect (van Katwijk and Wladimiroff, 1991). Sorensen et al. demonstrated a significant increase in the UA S/D ratio in the upright posture compared to the left lateral posture in pregnancies with hypertension but not in pregnancies with normotension; however, they did not include the supine posture in their protocol, which makes it challenging to contextualize these results (Sorensen et al., 1992).

In another study by Ryo et al. (N = 202), an abnormal UA RI, defined as being greater than the 90th percentile, in the supine posture at 35–37 weeks was found to have a sensitivity and positive predictive value (PPV) of 53% and 43%, respectively, for subsequent abnormal outcomes including small-for-gestational-age (SGA), fetal distress, and pregnancy-induced hypertension. Sensitivity and PPV declined precipitously for an abnormal UA RI in the left lateral posture at the same gestation and for both postures at an earlier gestation (27–29 weeks). These findings may lend support to clinical use of the supine posture in late pregnancy combined with UA Doppler velocimetry as a “stress test” for identifying at-risk pregnancies (Ryo et al., 1998).

Discrepancy in the aforementioned studies may be due to methodological differences. That said, while all the studies generally described their methods for obtaining Doppler measurements, the older studies provided less information than the newer studies. Most reported the ultrasound manufacturer, probe type and frequency, number of sonographers, timing and averaging of multiple measurements, and performing measurements in the absence of fetal body and breathing movements, with slight differences in these parameters, but only one study reported specifics of the criteria used in obtaining the measurements (e.g., keeping the insonation angle as close to zero as possible). Furthermore, most studies attempted to minimize variability by using one sonographer to make all the measurements. We submit, however, that differences and variability in anatomy and physiology between individuals may also contribute to the discrepancy seen in the literature. Similar to how some pregnancies are affected by SHS and some are not, it is possible that a supine-related increase in placental resistance follows the same individual variation. Such duality might be a result of various other contributing and confounding factors that impact the immediate uteroplacental environment and should be accounted for in future research. For example, polyhydramnios, supine posture, obesity, and sleep apnea all increase intra-abdominal pressure. Insightful in vitro models have been developed for states of increased external pressure on the placenta (Karimu and Burton, 1994; Kranenburg-Lakeman et al., 2001). Polyhydramnios is a state of increased pressure on the placenta from the fetal side, and SHS is a state of increased pressure on the placenta from the maternal side via increased venous pressures in the intervillous space from downstream occlusion of the IVC by the gravid uterus in the maternal supine posture. These models have demonstrated that fetal-placental capillaries are elastic, deformable, and compressible (Karimu and Burton, 1994), and these capillaries have nonlinear perfusion pressure-dependent resistance (Kranenburg-Lakeman et al., 2001). Notably, increased external pressures on the placenta increases placental bed resistance as seen by the fetus but only at the lower side of the pressure-flow curve (Kranenburg-Lakeman et al., 2001). That is, at lower perfusion pressures (such as would occur during fetal hypotension), external pressure on the fetal vessels in the placenta may overcome the intraluminal pressure of some of the fetal vessels, collapsing them, and causing the placental bed resistance to forward flow to increase. This may indicate that fetuses prone to hypotension are more vulnerable to the mechanical effects of external pressure on the placenta such as that encountered in polyhydramnios or SHS, which may occur, for example, in a fetus with a cardiac arrhythmia and duodenal atresia.

Indeed, Zun et al. recently employed advanced MRI techniques showing that fetuses with congenital heart disease are particularly affected by maternal posture as demonstrated by significantly lower global placental perfusion in the supine posture compared to the lateral side-lying posture (Zun et al., 2017). It is likely that compression of the placental capillaries occurs when the pressure in the intervillous space (maternal side) exceeds the pressure in the capillary lumen (fetal side), which results in an increase in the capillary resistance—recall that for pipe flow, resistance to flow is inversely proportional to the fourth power of the vessel’s radius by Poiseuille’s equation (Karimu and Burton, 1994). Overall, this effect may also contribute to the variable changes seen in placental resistance and UA flow parameters in the maternal supine posture.

In a recent study employing functional MRI at 34–38 weeks, Couper et al. found that maternal supine posture results in a 6.2% reduction in oxygen transfer across the placenta (p = 0.038), an 11.2% reduction trend in oxygen delivery to the fetus (p = 0.060), and a 4.4% reduction trend in fetal oxygen saturation (p = 0.079) in comparison to the lateral posture (Couper et al., 2021). Corroborating these findings, Abaci Turk et al. found significantly larger increases in global placental R2* (a reflection of blood oxygen saturation levels) in the maternal supine posture compared to the left lateral posture with maternal hyperoxygenation via 100% fraction of inspired oxygen (FiO₂) non-rebreather face mask for 5 minutes using a technique called BOLD (blood-oxygen-level-dependent) MRI in uncomplicated pregnancies between 27–36 weeks (Abaci et al., 2020). The authors suggested these results were most consistent with the sigmoidal nature of the oxygen-hemoglobin dissociation curve: for a given change in the partial pressure of oxygen (pO₂), there is a greater absolute change in oxy-hemoglobin percent saturation when starting at a lower pO₂ (when supine) than when starting at a higher pO₂ (when non-supine). Furthermore, it is well known that maternal arterial pO₂ is consistently lower in the supine posture than in the left lateral posture (Dunietz et al., 2020). Taken together, this indicates a significantly lower blood oxygen saturation in the placenta and umbilical cord in the maternal supine posture compared to the left lateral posture.

The effect of maternal posture on fetal heart rate (FHR) is debatable and the literature has not reported consistent results, largely related to differing study methodology. Indeed, a recent Cochrane review on cardiotocography (CTG) (assessment of FHR pattern and its temporal relationship to uterine contractions) by Alfirevic et al. urged future research to address the potential contribution of maternal supine posture to adverse outcomes and whether maternal posturing could be used to reduce adverse neonatal outcomes (Alfirevic et al., 2017). Regarding studies investigating maternal posture and the FHR in the third trimester in singleton pregnancies with intact membranes, we found that six of these studies were large, with over 100 participants in each study. Of these, one reported that posture makes no difference, one reported that supine posture was favorable, and four reported that supine posture was unfavorable. We discuss these six reports first followed by several smaller studies.

In a randomized controlled trial (RCT), Moffat and van den Hoff (N = 573) reported no significant difference in the incidence of non-reactive CTG and time to CTG reactivity (using the Dawes-Redman criteria) between the three postures they studied: semi-recumbent posture (semi-sitting with the upper body elevated at 30) with 45 left lateral tilt (wedge placed under the right hip and pelvis), with a 45 right lateral tilt, or with no tilt (Moffatt and van den Hof, 1997).

Nathan et al. (N = 108) completed a study in which they performed two 10-min CTG’s in two different postures: 1) semirecumbent (elevation of upper body not specified) and 2) left lateral recumbent (Nathan et al., 2000). Independent of order, there was more 10-min CTG non-reactivity in the left lateral recumbent posture than in the semirecumbent position. On a logistic regression, the first 10-min CTG and the left lateral recumbent starting posture independently predicted non-reactive CTG (Nathan et al., 2000). We find these results, namely, that the supine posture was favorable and the left lateral posture was unfavorable, to be paradoxical and less helpful since the CTG duration used in this study is not typically used in clinical practice.

Tamás et al. (N = 106) provided unique insight into FHR tracing morphology. The authors demonstrated a statistically significant increase in the number of FHR accelerations (25%) and overall and short-term FHR variation (20% each) and a decrease in basal FHR (3.5%) with the left lateral posture in comparison to supine (Tamás et al., 2007). There was no difference in FHR decelerations between the postures. Significant correlations were found between FHR short term variation and maternal CO in the supine posture and between the number of FHR accelerations and maternal stroke volume and CO in the left lateral posture. When maternal posture changed (from supine to left lateral), the resulting change in stroke volume (12% increase) correlated significantly with FHR overall variation and short term variation. Aluş et al. (N = 408) added to this work by considering more maternal postures (Aluş et al., 2007). They randomly assigned participants to complete a 20-min CTG in one of four postures: 1) seated upright, 2) semi-fowler, 3) supine, and 4) supine-left. The CTG reactivity (≥2 fetal movements associated with a ≥15 bpm acceleration lasting ≥15 s) was significantly lower when supine (69%) compared to any of the three other postures. The semi-fowler posture yielded the highest CTG reactivity (85.3%). Similar to Moffat and van den Hoff (Moffatt and van den Hof, 1997), Aluş et al. did not find a significant difference in the time taken to reach CTG reactivity between the four maternal postures. Cito et al. (N = 368) highlighted the importance of factoring in the effect of gestational age when investigating the impact of maternal posture on FHR (Cito et al., 2005). In their study, the time taken to achieve a reactive CTG (defined as three large FHR accelerations) was significantly decreased as pregnancy advanced (from <37 weeks, 37–39 weeks, and >39 weeks) when the CTG was completed during sitting or walking (Cito et al., 2005). This time, however, was unchanged with advancing pregnancy when the CTG was completed in the reclining posture. After 39 weeks, there were significantly more FHR variable decelerations in the reclining posture when compared to sitting and walking. Finally, and most recently, Ibrahim et al. (N = 118) reported increased FHR, accelerations, and fetal movements in the left lateral posture followed by semi-fowler, compared to supine (Ibrahim et al., 2021). No differences, however, were found between postures for FHR variability and CTG reactivity.

A small but unique study by Ellington et al. (N = 25) reported no significant differences in FHR (nor UA flow morphology) between the various tilted postures they tested including 1) supine, 2) supine with 5 (operating table angle) right lateral tilt, 3) supine with 10° right lateral tilt, 4) supine with 5° left lateral tilt, 5) supine with 10° left lateral tilt, and 6) supine with 10° left lateral tilt with a 10 cm wide wedge (rolled blanket) placed under the right hip (Ellington et al., 1991). Their study was completed in pregnancies between 25–40 weeks, however, which may have diluted the impact of compressional effects seen at more advanced gestations. Furthermore, others have shown that 15° of left lateral tilt is required to relieve physiologic effects of aortocaval compression by the gravid uterus and restore maternal CO (Lee et al., 2012). Armstrong et al. (N = 25) also reported null findings regarding the effect of maternal posture on FHR (and UA PI) in non-laboring, uncomplicated, term pregnancies (Armstrong et al., 2011). They considered four postures: supine with 15° left lateral tilt, sitting with neck and hips flexed, left lateral, and right lateral. Dennis et al. (N = 30) also found that maternal posture did not affect FHR in their sample of non-laboring, uncomplicated, term pregnancies (Dennis et al., 2018a). They considered three postures: 1) left lateral decubitus, 2) ramped upper-body with left lateral pelvic tilt (wedge), and 3) ramped upper body alone. Of note, they also found no difference in maternal CO in each posture, so a null effect of these maternal postures on FHR is not unexpected.

Swansburg et al. investigated pregnancies with preeclampsia (N = 9) at 32–40 weeks and used normotensive pregnancies (N = 18) as comparators (Swansburg et al., 2005). When presented with orthostatic stress (i.e., from lying to standing), parasympathetic tone decreased in both groups, but the normotensive group had more spontaneous FHR accelerations. These results indicate that fetuses in pregnancies with preeclampsia may be affected during orthostatic challenges, and this effect could be a result of autonomic dysfunction coupled with intravascular volume contraction seen in preeclampsia.

Finally, we found two studies reporting on FHR with regard to prone posture. Dennis et al. studied pregnancies with preeclampsia (N = 15) and healthy normotensive comparators (N = 50) at term (Dennis et al., 2018b). They found no difference in FHR in the prone posture (resting on a specially designed pillow with a center hole that supports the gravid uterus) compared with the left lateral posture. Note, however, that they did not actually measure the FHR when the participant was prone and only measured it immediately after the participant returned to the left lateral posture after resting in the prone posture for 5 minutes. Oliveira et al. (N = 33) also reported no differences in FHR between prone (resting for 6 minutes on a specially designed stretcher with a center hole allowing the gravid uterus to pass through it) and non-prone postures (included fowlers, supine, and left lateral) (Oliveira et al., 2017).

Overall, the literature on FHR suggests that it is impacted by maternal posture, specifically the supine posture seems to be unfavorable. More work, however, remains to be done as complete consensus has not yet been reached.

Fewer studies reported on fetal cerebral circulation, but blood flow in the brain is clearly an integral part of fetal neurological development. Khatib et al. (N = 23) studied low-risk pregnancies at 36–40 weeks and found that the middle cerebral artery (MCA) PI and the MCA peak systolic velocity (PSV) both decreased significantly after changing to the supine posture (assessed after 15 min of rest) from the left lateral decubitus posture (also assessed after 15 min of rest) (Khatib et al., 2014). The authors did not calculate the change in the cerebroplacental ratio, which we calculated for reference: the cerebroplacental ratio decreased from 2.0 (54th percentile) in the left lateral decubitus posture to 1.73 (30th percentile) in the supine posture. Silva et al. (N = 20) did a similar study at the same gestational age range (Silva et al., 2017). They reported that the MCA PI in the left lateral posture (mean 1.7) was significantly reduced at 5 minutes after assuming the supine posture (mean 1.4). The findings of these studies suggest that the maternal supine posture may present a physiological stress sufficient to induce fetal adaptation via cerebral redistribution (also known as the “fetal brain sparing phenomenon”). In addition to cerebral hemodynamics, maternal posture may also affect fetal cerebral oxygenation. Aldrich et al. (N = 14) found a significant decrease in the mean concentration of cerebral oxyhemoglobin and mean cerebral oxygen saturation in the fetus in the maternal supine posture compared to left lateral in term pregnancies in uncomplicated labor with effective epidural analgesia (Aldrich et al., 1995). That said, there was no significant difference in fetal mean concentration of deoxyhemoglobin and fetal cerebral blood volume.

Recently, Stone et al., in a first-of-its-kind study, presented good evidence that maternal posture affects fetal behavioral state (FBS), fetal heart rate variability (fHRV), and mean fetal heart rate in a manner where the fetus resorts to a protective, quiescent, low oxygen consuming state when the mother assumes the supine posture. This phenomenon is likely due to reduced uteroplacental perfusion and subsequent mild hypoxic stress induced by this posture (Stone et al., 2017). In the supine and right lateral postures, there is more FBS 1F (quiet sleep, fetal quiescence) compared to the left lateral posture. When in the supine and semi-recumbent postures, there is less fHRV, less FBS 4F (active awake), and an FBS state change is more likely to occur when compared to lateral postures (Stone et al., 2017).

While fetal growth is less easily observed than measuring the FHR or other fetal blood flow indices, Hohmann and Künzel hypothesized that birthweight is related to the maternal mean arterial blood pressure (MAP) response to orthostatic stress in late pregnancy, namely, that pregnancies in which MAP falls upon standing upright (“orthostatic hypotension”) would give birth to smaller infants (Hohmann and Künzel, 1991). Indeed, they found a linear relationship between the change in MAP in response to orthostatic stress in late pregnancy and birthweight. In other words, birthweight was directly related to the magnitude and direction of the change in MAP with standing in late pregnancy. We return to the impact of maternal posture on fetal growth in the Activities section below.

Approximately one-third of pregnancy is spent asleep. As such, sleep during pregnancy is a burgeoning area of research (Kember et al., 2023). We found that studies of maternal sleeping posture centered around three outcomes—placenta site, fetal growth, and late stillbirth—which we review in detail here. Other outcomes, such as the association between maternal sleeping posture on fetal occiput positioning in utero (Matsuo et al., 2007), had only one study and are not discussed further.

As we have previously highlighted that approximately one-third of pregnancy is spent asleep, we must also acknowledge that approximately the same amount of time is spent working. However, compared to the volume of literature relating to maternal sleeping posture, our search uncovered a much greater volume pertaining to maternal posture at work. As such, we focus our discussion on metaanalyses and systematic reviews that have been conducted pertaining to this literature and have divided this into the impact on maternal posture on pregnancy outcomes 1) prior to fetal viability (i.e., prior to approximately 23 weeks gestation) and 2) after fetal viability.

The landscape of research focusing on the impact of physical activity on fetal outcomes is vast. Historically, recommendations were made to reduce physical exercise during pregnancy out of fear there would be negative consequences such as pregnancy loss or reduced placental circulation (Barakat et al., 2008). It is now generally well accepted, however, that physical exercise during pregnancy is important for promoting maternal health. The impact of specific exercise postures during pregnancy on fetal outcomes remains controversial and understudied. This section narrows in on studies that evaluated the impact of maternal posture during exercise on fetal birth outcomes.

A systematic literature review by Schlüssel et al. evaluated 37 studies and focused on occupational physical activities, including housework, and leisure exercise, and their impact on both maternal and fetal outcomes (Schlüssel et al., 2008). Very few studies investigated the impact of physical activity during pregnancy on fetal development, though it appears that light and moderate physical activity is not a risk factor for poor fetal outcomes. There may be an association, however, between specific activities such as prolonged standing or climbing stairs and outcomes such as SM, low birth weight, and PTB. It was concluded that many gaps exist in this field and more research is needed to elucidate the trends more clearly and identify the safety of specific exercises (Schlüssel et al., 2008).

Due to the theoretical concern of supine positioning impacting maternal hemodynamics, the effect of supine exercise on fetal outcomes (such as low birth weight, SGA, and fetal demise) was reviewed systematically by Mottola et al. (Mottola et al., 2019) The review evaluated seven “low” to “very low” quality studies which ultimately concluded insufficient evidence exists to suggest whether maternal supine exercise during pregnancy is associated with adverse fetal birth outcomes (Mottola et al., 2019). Interestingly, a prospective study conducted by Polis, Gussman, and Kuo report the maternal and fetal effects of 26 yoga postures, each held for 2 min in 25 pregnancies in the late third trimester (35 weeks and 0 days through 37 weeks and 6 days) (Polis et al., 2015). Vital signs, pulse oximetry, uterine tocometry, and FHR were measured continuously through all postures and remained normal for all participants for the duration of the study. The participants did not experience any decreased fetal movement, contractions, leakage of fluid, or vaginal bleeding in the 24-h follow up. Even postures suspected to have a deleterious impact on the maternal or fetal physiology, including supine postures (Child’s Pose, Corpse Pose, Happy Baby Pose), did not cause any changes in normal monitoring. All postures were well tolerated and held longer than is done traditionally in yoga classes suggesting that yoga and many common supine yoga postures may be safe for the mother and fetus in the third trimester (Polis et al., 2015).

Avery et al. investigated changes in maternal HR, FHR, and maternal BP during moderate strength training conditioning exercises performed during two laboratory exercise sessions in 12 healthy pregnancies during the third trimester compared to non-pregnant controls (Avery et al., 1999). They found that the FHR baseline remained normal during and after maternal strength conditioning exercises. While in the sitting posture, no fetal bradycardia or FHR decelerations were observed during exercise and a very small number of decelerations were observed post-exercise. Moderate fetal bradycardia and FHR decelerations were observed occasionally while in the supine posture with 30° of upper body elevation before, during, and after exercise suggesting this posture may not be safe in the third trimester due to redistribution of blood flow during exercise. The increased frequency of FHR decelerations, however, was not statistically significantly compared to when the participants were in the sitting posture. The training may lead to increased fetal wakefulness as demonstrated by significant increases in frequency of FHR accelerations during double leg exercises in the seated posture (Avery et al., 1999). Overall, the results do not suggest that maternal or fetal health is significantly compromised during strength conditioning exercise in the third trimester, but the supine posture with 30° of upper body elevation might be best avoided to avoid potential fetal compromise.

Weng, Lee, and Chien found that reduced sitting time in late pregnancy had a negative effect on SGA (Weng et al., 2020). While this may suggest a protective effect of sitting, a reduction in sitting may also reflect increased participation in activities which negatively impact the fetus such as prolonged standing. Fazzi et al. conducted a systematic review to assess the impact of time spent in sedentary behaviors during pregnancy and how this may impact fetal outcomes (Fazzi et al., 2017). They found conflicting associations between sedentary behavior and birthweight, with some studies suggesting increased sedentary behavior is linked to larger newborn abdominal circumference and macrosomia, while other studies suggested lower birth weight was associated with increased sedentary behavior.

Overall, the great heterogeneity in exercises and outcomes as well as the volume and quality of studies limits the ability to draw conclusive advice around specific postures during exercise and the impact on the fetus. Higher quality methodology and increased investigation is needed to better elucidate the effect of various exercise postures and prolonged sitting and standing on fetal outcomes.

We found few studies of maternal posture during driving and transportation and fetal physiology. In a study of pregnancies by Kromka-Szydek et al., 90% of participants reported sitting during travel in a car, bus, and tram, and 24%, 18%, and 8% of participants reported enhanced fetal movements in these modes of transportation, respectively (Kromka-Szydek et al., 2018). The investigators hypothesized that more intense fetal movements while riding in a car, in which sitting posture is standard, are a direct result of more exposure to vertical vibratory accelerations (transmitted through the seat and backrest) within the range of resonance frequencies of vital organs in the abdominal cavity (Kromka-Szydek et al., 2018). A study by Irannejad Parizi et al. looked at the forces applied to the gravid uterus and fetus (in head-down, occipito-anterior position) while riding in a passenger vehicle and experiencing sudden acceleration by passing over a speed-bump (Irannejad Parizi et al., 2021). Using their model, they recommend that a driver should not hit a standard speed-bump (0.5 m wide and 0.12 m tall) at 42 km/h or faster in order to avoid risk of fetal head injury. They advise hitting such speed-bumps under 25 km/h, and their model can be used to determine speed-bump dimensions that will avoid fetal injury at any desired vehicle speed.

Research shows that while the majority of pregnant patients believe that questions relating to coitus during pregnancy are important and should be discussed with their healthcare provider, these questions are only discussed with a minority (22%–29%) of patients (Bartellas et al., 2000; Uwapusitanon and Choobun, 2004), and a significant proportion of patients feel uncomfortable bringing up the topic themselves (Bartellas et al., 2000). In early pregnancy, heterosexual patients’ concerns center around coitus causing SM (Uwapusitanon and Choobun, 2004) and then give way to increasing concerns about coitus causing PTB and preterm prelabor rupture of membranes (PPROM) (Bartellas et al., 2000). With regard to maternal posture during sexual intercourse in pregnancy and its potential impact on fetal physiology, we found two reports. While Bartellas et al. did not report on fetal outcomes they did find a significant reduction in the reporting of “man on top” posture as gestation advances (Bartellas et al., 2000). These results provide context for a study by Ekwo et al. where only the missionary position was significantly associated with PPROM (OR 2.40, 95% CI 1.16–4.97) and PTB without PPROM (OR 1.82, 95% CI 1.02–3.25). Furthermore, no posture was found to be associated with term prelabor rupture of membranes. While these data are limited, they indicate that clinicians should discuss coitus with their antenatal patients and consider recommending alternatives to maternal supine posture (inferencing military position) as pregnancy progresses.

In terms of maternal hand positions for introducing the breast to the infant, two that have been studied are palmar grasp and scissor grasp, and these two hand positions are no different when evaluated relative to infant milk intake (Altuntaş and Ünsal, 2019). As for maternal posture during breastfeeding, lactation experts suggest that maternal posture is an important component of breastfeeding education, reducing breastfeeding problems, and enhancing long-term breastfeeding (Henderson et al., 2001; Mbada et al., 2013). A recent meta-analysis, however, has demonstrated that the laid-back posture is beneficial for mitigating lactation-related nipple problems and is conducive to correct latching of the infant (Wang et al., 2021). An RCT of laid-back versus side-lying posture for breastfeeding following cesarean section found no statistically significant differences in breastfeeding outcomes related to the infant, but participants were more satisfied with the side-lying posture (Puapornpong et al., 2017), which may be related to experiencing less fatigue in this posture (Milligan et al., 1996).

A 2013 meta-analysis by Lawrence et al. has shown a reduction in NICU admissions (RR 0.20, 95% CI 0.04–0.89) in patients without epidural anesthesia who alternated between the upright (walking, sitting, standing and kneeling) and recumbent (supine, semi‐recumbent, and lateral) postures in the first stage of labor compared to those who were in the recumbent posture only (Lawrence et al., 2013). This conclusion, however, was based on the result of only one RCT of 200 participants. For patients with an epidural, there was no difference in fetal outcomes between the same postural groups (upright and recumbent versus recumbent only) (Lawrence et al., 2013). In an older meta-analysis (2004) of patients with an epidural in the first stage of labor, Roberts et al.‘s analyzed five trials (N = 1,161) and concluded that there was no significant difference in Apgar scores between patients who ambulated compared to those who remained recumbent (Roberts et al., 2004).

A 2017 updated meta-analysis by Gupta et al. has shown a reduction in abnormal FHR patterns (RR 0.46, 95% CI 0.22–0.93, 2 trials, N = 617) in any upright posture in the second stage in patients without an epidural compared to supine postures regardless of gravidity (Gupta et al., 2017). There was no difference in the number of infants admitted to NICU (RR 0.79, 95% CI 0.51–1.21, 4 trials, N = 2,565 infants), but the quality of evidence was low. In patients with an epidural, Walker et al.’s updated meta-analysis in 2018 reported fewer infants born with low cord pH in the upright group (RR 0.43, 95% CI 0.20–0.90, 2 trials, 3,159 infants, moderate-quality evidence) but did not identify any clear differences in abnormal FHR patterns requiring intervention (RR 1.69, 95% CI 0.32–8.84, one trial, N = 107), or admission to NICU (RR 0.54, 95% CI 0.02–12.73, one trial, N = 66 infants) (Kibuka and Thornton, 2017; Walker et al., 2018). Note, however, that the quality of evidence for the latter two outcomes was low, with more research required, particularly for additional fetal outcomes (e.g., Apgar scores, ventilatory requirements, perinatal death). An earlier meta-analysis (2005) of two trials (N = 281) by Roberts et al. reported that data were insufficient pertaining to infant outcomes and the possible effect of maternal posture (upright vs recumbent) in the second stage of labor with an epidural (Roberts et al., 2005). A 2020 meta-analysis by Dokmak et al. including seven RCT’s (N = 1,219) reported no difference in fetal outcomes for the maternal squatting posture (compared to supine) during the second stage of labor (Dokmak et al., 2020). They were unable to stratify for use of epidural anesthesia.

In summary, for patients without an epidural, upright postures in the first and second stage may confer fetal benefit, whereas data and postures are limited for patients with epidurals and further investigation is required. Note that the bulk of the evidence is regarding upright posture versus supine posture—few trials and no meta-analyses looked at supine versus a lateral, side-lying position in patients with epidural anesthesia, which may be beneficial since the majority of patients, in our region and demographic, opt for epidural anesthesia, which frequently limits posture options.

We found one meta-analysis (2010) on the influence of gravity (not maternal posture) on the newborn during the third stage of labor (the time from birth of the infant until delivery of the placenta) (Airey et al., 2010). However, no studies met the author’s inclusion criteria, and they concluded that large RCT’s are required in order to determine the effect of gravity on placental transfusion and fetal outcomes during vaginal birth and CS birth. A later non-inferiority RCT (2014) by Vain et al. concluded that the volume of placental transfusion is not affected by gravity (i.e., the position of the neonate prior to cord clamping) and that the neonate may be safely held on the mother’s abdomen or chest immediately after birth.