Pregnancy fundamentally transforms a woman’s life, bringing about a multitude of physiological and anatomical changes necessary for the developing fetus. Respiratory mechanics experience significant changes as metabolic demands increase and design evolves, representing some of the most notable alterations (1, 2). The second trimester, occurring between weeks 13 and 26, represents a pivotal period where these physiological adaptations experience notable changes (1, 2). Vital capacity is an essential factor in respiratory function as it assesses the maximum amount of air that can be exhaled after taking a deep breath (3). Various factors, including body posture, can potentially impact vital capacity, a key indicator of lung health (3). It is important for pregnant women to understand how different body positions affect their vital capacity. This can help them improve their respiratory well-being during pregnancy (3, 4).

A study by Patel et al. (5) investigated the impact of different body postures on respiratory function during pregnancy, focusing specifically on dyspnea, a common issue caused by physiological changes. The researchers evaluated Peak Expiratory Flow Rate (PEFR), a key measure of lung function, among 72 participants between 14 and 27 weeks of gestation while in various relaxation postures. The findings revealed that relaxed standing, sitting, or high side-lying positions were associated with decreased PEFR levels, whereas front-lean standing, or forward lean sitting positions resulted in the highest PEFR values. Notably, upright postures consistently yielded higher PEFR results compared to recumbent positions, suggesting that adopting specific body positions may benefit respiratory health during pregnancy (5). Evidence suggests that expecting mothers can alleviate respiratory difficulties by adopting a forward-leaning posture while standing, sitting, or lying down. This significant finding offers crucial guidance for combating dyspnea and improving overall comfort for pregnant women (5–7).

Another study by Harirah et al. (8), examined the effects of gestational age and maternal position on peak expiratory flow rates (PEFR) in pregnant women. By recording PEFR values in standing, sitting, and reclined positions throughout pregnancy and after delivery, the researchers observed a substantial decline in PEFR with increasing gestational age (8). This decline was more pronounced in the supine position compared to standing or sitting postures. Notably, PEFR levels reached approximately 71.9% of early pregnancy values after childbirth. The study underscores the importance of considering maternal position and gestational age when interpreting PEFR measurements, particularly in pregnant women with asthma (8).

During pregnancy, the organs’ organization and function undergo significant changes. During the first trimester of pregnancy, the uterus is located in the pelvic cavity. However, as the pregnancy develops during the second and third trimesters, the uterus moves into the belly, significantly altering the size of the pelvic and abdominal cavities (9). The abdominal organs grow and reorganize, lengthening the diaphragm, a vital muscle for breathing. When stretched, the diaphragm thins in the opposite direction to the tension. The Poisson’s ratio notion is appropriate in this situation. The expanding uterus’s evenly rising pressure on the diaphragm’s abdominal surface causes it to lengthen when in posture (10, 11).

The rising abdominal pressure also suggests an increased inspiratory load on the diaphragm during pregnancy. The expanded diaphragm and the greater load indicate that the muscle contracts eccentrically during inhaling (3, 4). The diaphragm may ultimately get stronger and more conditioned because of these eccentric contractions that take place during normal breathing; this is vital during childbirth, when the diaphragm is crucial (4, 9). It is crucial to assess respiratory function in pregnant women, especially during the second trimester. We aimed to bridge the gap in knowledge by conducting thorough research on the impact of body position on slow vital capacity (SVC) in pregnant women during their second trimester.

Our study examined 136 s-trimester pregnant women, to evaluate impact of sitting and standing positions on SVC. The majority of the participants were in their sixth month of pregnancy, with a mix of first-time and experienced mothers. Despite this diverse demographic, our analysis of SVC measurements in different postures—sitting versus standing—revealed no statistically significant differences. This suggests that body posture may not significantly impact lung function in this specific group of pregnant women. Our findings diverge from previous research by Patel et al. (5), which suggested that posture might influence breathing efficiency in pregnancy, with certain positions possibly aiding in alleviating respiratory discomfort. Our study’s contrasting outcome could be attributed to the different gestational stages of the participants. The majority of our participants were in the later stages of the second trimester, suggesting that the effect of gestational age on lung capacity might be more pronounced as pregnancy progresses.

Additionally, this study had a large group of participants who were not primigravida (58.8%), which significantly affects the results as primigravida are experiencing the pregnancy trimesters for the first time, having a heightened experience for every change in their body in comparison to non-primigravida participants who have already experienced pregnancy and are aware of the body changes to expect, thus, may have adapted accordingly. Another study conducted by Ruhighara et al. (18), which aimed to analyze the spirometry profiles among pregnant and non-pregnant African women reported that spirometry test values are lower in pregnant than in non-pregnant participants. However, the study also reported that the spirometry profile was higher in parous than in nulliparous women which aligns with our study findings (18).

The BMI is an essential variable that may impact the performance of the SVC during the pregnancy. However, our study did not yield any significant association between BMI and SVC in both positions. This indicates that BMI may not have a substantial impact on lung function in pregnant women during the second trimester. Our findings are consistent with previous studies that also supported this conclusion (19, 20). Additionally, the study also examined the variation in SVC among pregnant women at various gestational ages, with a particular emphasis on the fourth, fifth and sixth months of pregnancy. Based on our study, there was a significant difference in SVC between the fourth and sixth months of pregnancy in the sitting position. The sixth-month individuals had higher SVC values. On the other hand, no appreciable variations in SVC were noted in the standing position over the several pregnant months. Because of the cumulative physiological changes that occur as pregnancy progresses (4, 7, 19–21), our findings suggest that gestational age may have a more apparent effect on lung function in specific body positions.

While this study offers valuable insights, it is essential to acknowledge its limitations. The relatively small sample size of 136 participants in this study may limit the generalizability of the findings to the broader population of pregnant women. In addition, the exclusion of women with lung diseases or a history of smoking limits the application of findings, even if it is necessary. Further studies of such nature should examine these effects in a more diverse population. Moreover, potential confounding factors such as physical activity levels and pre-existing medical conditions. The impact of lifestyle factors, especially physical exercise, on respiratory function has been reported in the literature. For example, Leite et al. (22) found that physical activity modulates cardiovascular and metabolic responses during pregnancy, implying that it may also have an indirect impact on respiratory dynamics. Similarly, pre-existing medical disorders, even if not directly related to pulmonary function, may alter physiological adaptations during pregnancy (21). Furthermore, longitudinal studies tracking respiration changes in each trimester would provide a complete understanding and would be further elaborated by comparing multiparous women to primigravida women (4, 11). This research can help fine-tune advice for physicians and the pulmonary function technologists to conduct pulmonary function tests for pregnant women, especially in late second trimester, in a standing position.

Our study contributes significant insights into how body position affects a woman’s SVC during the second trimester of pregnancy. While a combination of primigravida and multiparous women were among our subjects, there were no statistically significant differences in SVC between sitting and standing positions. Furthermore, neither the standing nor the sitting positions showed a statistically significant correlation with BMI and SVC, indicating that BMI may not have a substantial impact on lung function in the second trimester of pregnancy. Conversely, the progression of SVC underwent significant changes from the fourth to sixth months of pregnancy when comparing the sitting position with the standing position. These changes can be attributed to the cumulative physiological transformation that occur as pregnancy advances.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The study (SP22J/128/08) was approved by the IRB committee of King Saud bin Abdulaziz University for Health Sciences. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

RaA: Conceptualization, Investigation, Resources, Supervision, Writing – original draft. SA: Data curation, Writing – original draft. WA: Data curation, Writing – original draft. ReA: Data curation, Writing – original draft. MA: Project administration, Supervision, Validation, Visualization, Writing – review & editing. ZA: Formal Analysis, Investigation, Methodology, Project administration, Software, Supervision, Validation, Visualization, Writing – review & editing.