Fetal growth restriction (FGR) is a major cause of increased risk of mortality and morbidity in infants occurring in 5%–10% of pregnancies with higher rates in low-income countries (1, 2). FGR is defined as ultrasound estimated fetal weight below 10th percentile with other factors included for diagnosis such as umbilical artery Doppler flow assessment, physiological determinants and neonatal features of malnutrition (3–5).

The causes of FGR are complex and can arise from several potential sources including maternal (hypertension, diabetes, preeclampsia, malnutrition, maternal age) (6), placental (placental dysfunction leading to poor blood supply and nutrient transfer) (7), and fetal (multiple pregnancy, congenital infections, genetics) (6). Placental insufficiency is the most common cause where a reduction in supply of oxygen and nutrients to the developing fetus results in a chronic hypoxic environment impacting normal growth and development of the fetus.

The chronic hypoxic environment that FGR fetuses are exposed to in utero particularly affects brain development. Lower grey matter and white matter complexity are major neuropathophysiological features of FGR which are shown to persist after birth (8). Changes in white matter microstructure are observed at 8–10 years of age in FGR compared to normally grown (NG) children (9). Adverse long-term neurological outcomes are reported in 24%–53% of FGR infants (10, 11), including language delays, learning and behavioral problems and cerebral palsy (CP) (12–14). Yet, the ability to identify FGR neonates at risk of adverse neurodevelopmental outcomes is lacking. Unlike for neonatal hypoxic-ischemic encephalopathy (HIE) where severity of injury is assessed using the Sarnat Grading Scale, there is no defined criteria at birth to score injury in FGR babies. Therefore, the likelihood to predict which newborn may have adverse outcomes is not clear and therefore interventions and support systems are commonly not implemented until developmental delays become evident years later. Due to evidence of positive impacts on CP infants with early physiotherapy intervention after birth (15), it is crucial to determine effective and efficient methods to identify FGR infants at risk of long-term adverse neurodevelopmental outcomes.

Studies have investigated the reliability of cranial ultrasounds, magnetic resonance imaging (MRI) and electroencephalography (EEG) to provide an indication of risk of adverse neurodevelopmental outcomes for FGR infants (16, 17). All three methods however, require further evidence for confirming the sensitivity of predicting long-term adverse neurodevelopmental outcomes in FGR. Cranial ultrasounds can identify brain structure abnormalities such as brain volume neuropathologies including intracranial hemorrhages and hydrocephalus (18). Although MRIs can detect more subtle structural changes such as white matter injury which has associations with cognitive and motor outcomes, the costs, resources and expertise required do not make it an ideal method for widespread implementation (19, 20). EEG studies have shown promise in identifying FGR infants at risk of adverse neurodevelopmental outcomes, however studies to date have utilized only visual interpretation of EEGs which relies on operator proficiency (17), rather than objective quantitative analysis.

Due to the limitations of these current methods for measuring subtle brain alterations, the search for sensitive predictors of long-term adverse neurological outcomes is ongoing. Blood biomarkers have the potential to provide an objective indication of risk of adverse brain outcomes. The diagnostic and prognostic utility of blood biomarkers to detect adverse brain outcomes is currently being investigated in adult and neonatal neurological disease states. There is evidence to suggest that alterations in the brain due to injury or disease are reflected in the blood. For example, alterations in proteins have been detected in blood samples from amyotrophic lateral sclerosis patients that distinguish these patients from healthy controls and other brain conditions (21–23). Furthermore, alterations in proteins present in blood of traumatic brain injury patients are correlated with adverse brain outcomes (24–26). Interest in blood biomarkers has grown with several recent studies examining blood biomarkers in FGR neonates. This systematic review examines the current literature to identify potential blood biomarkers that indicate risk of long-term adverse neurodevelopmental outcomes in FGR neonates with the ultimate goal to enable earlier interventions. We hypothesize blood biomarkers may indicate risk of adverse neurodevelopmental outcomes in FGR infants shortly after birth.

Excluding duplicates, 1,368 records were retrieved across the four databases searched (Figure 1). Only nine studies remained for full-text review after the initial title and abstract screening (Figure 1). Five of these studies did not include a control group of NG infants (31–35). These would have also been excluded for other reasons such as not meeting FGR definition (very low birth weight, preterm birth only), not assessing neurodevelopmental outcomes, or that no blood biomarkers were assessed. One study was excluded as it did not include neurodevelopmental assessments (36) and another was excluded as it reported only the presence of brain injury for the FGR cohort (37). Finally, one study was a twin study but was excluded as it did not separate the FGR twins from the normally grown pair for comparison (38).

This resulted in only one remaining study to be included for analysis.

To broaden our perspective, we considered whether any additional papers would be included if we altered our inclusion criteria to include small for gestational age (SGA) and FGR. Small for gestational age (SGA) differs from FGR in that it is solely defined by birthweight <10th percentile (3–5). We observed, however, that the studies included for full text review that included SGA cohorts either did not have a cohort of NG infants as controls or did not conduct a neurodevelopmental assessment >12 months of age for the NG cohort (31–37). Therefore, even when broadening the inclusion criteria to include SGA infants, our search of the current literature still only yielded one study that met all inclusion criteria.

Thirty-one FGR and 25 non-FGR or NG neonates were recruited for this study (39). FGR neonates were recruited consecutively, and NG neonates were gestational age-matched (±7 days) to the FGR neonates. FGR was confirmed after birth if birthweight <3rd percentile or, <10th percentile with abnormal Doppler study or abnormal cerebroplacental ratio. Mean gestational age at delivery was not statistically significantly different between NG and FGR neonates (262 days [standard deviation (SD) = 21] and 257 days (SD = 15) respectively). Gender differences were not reported. Birthweight and birthweight percentile were much lower in the FGR cohort compared to NG neonates as expected. Incidence of nulliparity, preeclampsia and severe preeclampsia were all significantly higher in the FGR cohort.

Both fetal umbilical arterial and venous blood samples were collected at the time of delivery and supernatant stored at −80°C (39). Serum S100B protein and neuron specific enolase (NSE) assays were performed to measure the protein levels in both sample types. These formed the four individual index tests examined in this study. At 2 years of age, developmental function was assessed using BSITD third edition (BSITD III), conducted as per the test manual.

This is the first systematic review to our knowledge of potential blood biomarkers for identifying risk of adverse neurodevelopmental outcomes in FGR. Only one study was identified as relevant to this systematic review despite ensuring our search terms were sufficiently broad to account for differences in terminology in the literature. This systematic review has the scientific merit of our search strategy returning key articles to inform a scientifically robust examination of the topic. If inclusion criteria were broadened more articles may be included, but this would negatively impact on scientific quality and impact as meta-analysis could not be conducted.

This search of the literature highlighted a number of gaps, limitations and differences in terminology that exist. Firstly, the search terms used were relatively broad to ensure these captured all potential publications investigating blood biomarkers for detecting risk of adverse neurodevelopmental outcomes in FGR infants. For example, some studies use “intrauterine growth restriction” (IUGR) to describe FGR (39), while other studies report SGA but also include cohorts that matched our definition of FGR (37). Another example of this was sufficiently capturing a variety of possible descriptions for “adverse neurodevelopmental outcomes”. Some articles investigated “neural injury” (39), while others investigated “brain injury” (34), or “white matter damage” (35). Although a number of different search terms were used as alternatives to broaden the search query, only nine publications were included in the full-text review, with only one meeting all inclusion criteria.

Only two blood biomarkers were considered in the identified study (39) which was deemed high quality using QUADAS-2. These results indicate the potential blood biomarkers have for indicating risk of adverse neurodevelopmental outcomes for FGR infants. However, as eight studies were excluded from this systematic review for multiple reasons, as mentioned above, this demonstrates the clinical literature lacks rigor. Preclinical studies are extremely important to answer research hypotheses as they can be undertaken in controlled environments. However, changing the approach of this review to include preclinical literature is not feasible as there are no studies to our knowledge that investigate blood biomarkers of brain injury in animal models of FGR. This review identifies the need for future studies to further investigate blood biomarkers for risk of adverse neurodevelopmental outcomes in FGR infants.

Neurodevelopmental outcomes measured at 2 years were not statistically significantly different between NG and FGR infants in the identified study (39). This finding does not align with the majority of studies demonstrating significant adverse long-term neurodevelopmental outcomes observed in FGR compared with NG infants (10–14). This may be due to the selection process for the NG group. What has been described here as the NG control group for Mazarico, Llurba (39) is more appropriately described in the study as a non-FGR group. Although this group is not classified as FGR, these infants have comorbidities such as 8% from preeclamptic pregnancies, 16% with respiratory distress syndrome, 28% neonatal morbidity and 32% neonatal admission (compared to 24%, 25.8%, 32.2% and 58% for FGR respectively) (39). The results indicate that the blood levels of NSE and S100B may indicate risk of adverse neurodevelopmental outcomes for all infants. This is an interesting and important finding, but also suggests a significant limitation of the study, that the non-FGR group also potentially has comparable rates of adverse neurodevelopmental outcomes and may not be a true representation of NG infants. Furthermore, a recent systematic review demonstrates the potential for NSE and S100B as biomarkers of injury in the newborn. They showed levels of both biomarkers correlate with unfavorable outcomes in HIE newborns (41). Although the paper concluded more studies are required to determine the sensitivity and specificity of these biomarkers, this further demonstrates the potential for these two biomarkers to predict adverse outcomes.

Four studies were excluded from the systematic review due to not meeting all the selection criteria, however some of their findings were of particular interest and may suggest other blood biomarkers to examine in future long-term studies.

The first of these studies considered correlations between cord blood gas and outcomes at 6–12 months in infants with low birthweight (31). Although the best single predictor for mental development index was pH of infant arterial blood gas, the best single predictor for psychomotor development index was gestational age (31). These findings demonstrate the need for research into blood biomarkers as indicators of risk of adverse outcomes to be broadened as current measures such as cord blood gases provide only limited insight.

Another study excluded after full-text review compared blood biomarker levels in FGR infants with brain injury to infants without brain injury on postnatal days 1–5 (34). Although increases in interleukin (IL)-6, IL-8, IL-10 and glial fibrillary acidic protein (GFAP) were observed in FGR infants with brain injury (34), it is not clear whether similar increases would be observed in cases of more subtle brain injury in FGR infants that are not currently detectable at birth.

A third study excluded from full text review identified increases in levels of NSE and ischemia-modified albumin (IMA) for SGA infants with brain abnormalities compared to SGA infants without brain abnormalities (37). These brain abnormalities were identified using cranial ultrasounds 12 h after birth (37). Although this finding is interesting, like the previous studies highlighted, it also does not provide evidence on whether these proteins indicate risk for adverse long-term outcomes in FGR infants.

Finally, a twin study where one twin was FGR and the other >25th percentile for birthweight, was ultimately excluded due to the NG sibling's data being combined with the FGR sibling (38). Correlations were identified between maternal metabolite levels of creatine, L-serine, L-arginine, and L-histidine with communication, first walking time, gross and fine motor function, and problem solving (38). With these findings indicating correlations between maternal metabolite levels and outcomes at 2 years of age even when FGR only impacts one twin, this demonstrates the potential for biomarkers to provide an indication of risk of adverse outcomes at birth.

The current literature identifies inverse relationships between blood biomarkers, NSE and S100B, measured at birth and neurodevelopmental outcomes at two years of age (39). This systematic review highlights the need for more research to determine blood biomarkers that indicate risk of adverse neurodevelopmental outcomes in FGR infants. Future studies assessing outcomes at school age would also be beneficial as cognitive and motor impairments are commonly observed at this age range, but unfortunately no FGR blood biomarker studies have assessed school age outcomes. Although there are many challenges for future research in this area, identifying blood biomarkers that indicate risk of adverse neurodevelopmental outcomes for FGR infants has the potential to vastly improve the lives of these infants, as up to 50% of FGR infants are reported to have adverse neurodevelopmental outcomes (10, 11). With notable improvements observed in infants with CP when interventions are implemented earlier (15), it is crucial to investigate effective and efficient methods to identify FGR infants at risk of long-term adverse neurodevelopmental outcomes in order to implement earlier support and interventions. This will enable these infants to have the best possible long-term neurodevelopmental outcomes.