What Is Fetal Growth Restriction (FGR) in Pregnancy?

Fetal growth restriction, commonly called FGR, is a pregnancy complication in which a baby fails to reach its expected size in the womb, usually defined as an estimated weight below the 10th percentile for gestational age combined with signs of compromised blood flow or nutrition. It affects roughly 5 to 10 percent of pregnancies worldwide and is one of the leading contributors to stillbirth and newborn illness. The condition stems from a web of overlapping problems, most often rooted in a placenta that cannot keep up with the baby’s demands, and it carries consequences that can extend well beyond birth for both the child and the mother.

What Goes Wrong in the Placenta

The placenta is the baby’s life-support system: it delivers oxygen and nutrients from the mother’s bloodstream and carries waste back out. For that exchange to work efficiently, the mother’s uterine arteries need to undergo a dramatic remodeling early in pregnancy. Specialized cells from the developing placenta invade the walls of these spiral arteries and widen them, converting them from narrow, muscular vessels into large, low-resistance channels. When that remodeling fails or falls short, the placenta never receives the blood supply it needs.¹ The result is a cascade of problems: the placenta becomes starved of oxygen, its tissue suffers oxidative damage, and it can no longer transport enough nutrients to the fetus.²

This is not usually the result of a single defect. Research increasingly describes FGR as a self-reinforcing loop in which impaired cell invasion, defective artery remodeling, immune dysfunction, and disrupted signaling molecules all feed into one another.³ That is part of why FGR can look so different from one pregnancy to the next: the underlying placental failure can range from mild to severe, and the timing of when it sets in shapes the baby’s trajectory.

Who Is at Higher Risk

A number of maternal factors raise the odds of FGR, and many of them are interconnected. Smoking is among the most well-documented. A large population-based study of over 17 million singleton births found a clear dose-response pattern: the more a woman smoked during pregnancy, the higher the rate of growth restriction. FGR occurred in roughly 3 percent of nonsmokers and climbed to 10 percent in the heaviest smokers.⁴ The encouraging finding was that women with preexisting high blood pressure who quit smoking before becoming pregnant brought their FGR risk back in line with nonsmokers who also had high blood pressure.

Hypertensive disorders, including chronic high blood pressure and preeclampsia, are independent risk factors. One study estimated that pregnancy-induced hypertension accounted for about 22 percent of preterm cases of small-for-gestational-age births.⁵ Other recognized risk factors include advanced maternal age, low pre-pregnancy weight, chronic kidney disease, autoimmune conditions, and certain infections. In a subset of cases, the problem lies not with the placenta or the mother but with the fetus itself. Among growth-restricted pregnancies with structural birth defects, chromosomal abnormalities have been identified in roughly 29 percent, with trisomy 18 being the most common.⁶

Early-Onset Versus Late-Onset FGR

Not all FGR behaves the same way. Clinicians increasingly divide the condition into two broad categories based on when it appears. Early-onset FGR, typically diagnosed before 32 weeks, is more often tied to severe placental disease and frequently co-occurs with preeclampsia. Late-onset FGR, diagnosed at or after 32 weeks, tends to involve a milder form of placental underperformance and is far more common.

A meta-analysis pooling data from 14 studies found strikingly different outcomes between the two. Pregnancies with preeclampsia were about four times more likely to be associated with early-onset FGR than with the late-onset form. Babies with early-onset FGR faced dramatically higher risks across the board: they were over 13 times more likely to need intensive care admission, about 10 times more likely to die in the perinatal period, and roughly 6 times more likely to score poorly on newborn health assessments.⁷ These differences are partly explained by the earlier gestational age at delivery, which means the baby’s organs are less mature, but the severity of placental disease itself also plays a role.

Late-onset FGR carries its own risks, though they are subtler and easier to miss. Because these babies often fall just below normal thresholds on ultrasound, the condition sometimes goes undetected until late in the third trimester. Despite being milder case by case, late-onset FGR accounts for a larger share of adverse outcomes simply because it is so much more common.

How FGR Is Detected and Monitored

Diagnosing FGR begins with serial ultrasound measurements of the baby’s head, abdomen, and femur to estimate fetal weight. A single measurement below the 10th percentile raises suspicion, but confirming FGR usually requires evidence that growth is slowing over time or that blood flow is abnormal. That is where Doppler ultrasound becomes essential.

Doppler studies measure the speed and direction of blood flowing through key vessels. Two are especially important: the umbilical artery, which reflects how much resistance the placenta is putting up against blood flow, and the middle cerebral artery in the baby’s brain, which reflects a protective reflex called brain sparing. When a fetus is not getting enough oxygen, its body redirects blood toward the brain and away from the gut and limbs.⁸ The ratio of flow in these two arteries, called the cerebroplacental ratio, has become a cornerstone of FGR monitoring.⁹ A low ratio signals that the baby is under stress and redirecting blood to protect its brain, which helps doctors decide when and how urgently to deliver.

Beyond Doppler, blood-based biomarkers are showing promise. One of the most studied is the sFlt-1/PlGF ratio, which measures circulating proteins involved in blood vessel formation. In high-risk pregnancies, this ratio has shown strong accuracy for predicting early-onset FGR and preeclampsia, with one prospective study reporting 100 percent sensitivity and about 81 percent specificity when measured between 24 and 28 weeks.¹⁰ The ratio also correlates with the severity of Doppler abnormalities, meaning it may help clinicians gauge how quickly a pregnancy is deteriorating even between ultrasound appointments.¹¹

Can FGR Be Prevented

The only intervention with solid evidence behind it is low-dose aspirin, started early in pregnancy. Meta-analyses show that aspirin reduces the risk of FGR by about 18 percent when initiated before 16 weeks of gestation, and it also lowers the chances of preeclampsia and preterm birth.¹² The catch is that it works best when started early and given at an adequate dose, and it depends on identifying at-risk women in the first place. Current screening tools combine a woman’s medical history, blood pressure, uterine artery Doppler at the first-trimester scan, and sometimes blood biomarkers to flag those who might benefit.

Other potential preventive therapies are still experimental. Low-molecular-weight heparin, statins, nitric oxide donors, and growth-factor-based treatments have all been explored in research settings, but none has yet shown reliable enough benefit to enter routine clinical practice.¹³ Dietary approaches targeting the nitric oxide pathway, such as L-arginine supplements, have likewise been disappointing in clinical trials, partly because the amino acid gets broken down in the gut and liver before it can do much good.¹⁴ For now, the most reliable prevention strategy remains identifying modifiable risk factors, especially smoking and uncontrolled blood pressure, and addressing them before or early in pregnancy.

Deciding When to Deliver

There is no treatment that reverses FGR once it has set in. The only definitive resolution is delivery, which creates a difficult balancing act: every extra week in the womb allows the baby’s lungs and brain to mature further, but a severely compromised placenta can deteriorate to the point where staying puts the baby at risk of stillbirth. The decision depends on gestational age, how abnormal the Doppler findings are, whether the mother has additional complications like preeclampsia, and the resources available in the neonatal unit.¹⁵

For late-onset FGR detected near term, management protocols generally stratify pregnancies by severity. Low-risk cases may be monitored closely and delivered between 40 and 41 weeks, while higher-risk cases are typically delivered between 37 and 38 weeks.¹⁶ In early-onset FGR with severe Doppler abnormalities, such as absent or reversed blood flow in the umbilical artery, delivery may be recommended as early as 28 to 34 weeks, depending on how quickly the fetus appears to be deteriorating. These are some of the hardest judgment calls in obstetrics, and they are made case by case with frequent monitoring.

What Happens After Birth

Growth-restricted newborns face a distinct set of immediate health challenges. Hypoglycemia, or dangerously low blood sugar, is one of the most common and has been identified as a hallmark complication.¹⁷ These babies have minimal fat reserves and limited glycogen stores, so their blood sugar can drop rapidly after birth. Hypothermia on admission to the nursery is also frequent, affecting over 44 percent of preterm growth-restricted infants in one cohort, compared with about 15 percent of similar-sized preterm babies who were not growth-restricted.¹⁸ Necrotizing enterocolitis, a dangerous bowel condition, was about six times more common in growth-restricted infants in that same study.

These short-term complications are managed in neonatal intensive care units, and most affected babies recover. But FGR also leaves a longer shadow. The developmental origins theory, sometimes called the “thrifty phenotype” hypothesis, suggests that a fetus deprived of adequate nutrition in the womb undergoes metabolic reprogramming that was originally adaptive for survival but becomes harmful in an environment of plentiful food after birth. This sets up a trajectory toward obesity, insulin resistance, and eventually cardiovascular disease.¹⁹ The pattern is especially strong when rapid “catch-up growth” occurs during infancy and early childhood.

Neurodevelopmental Risks for the Child

Growth restriction in the womb also affects brain development. The brain-sparing reflex described earlier, in which the fetus redirects blood toward its brain, is a protective mechanism, but it is not a perfect one. Chronic oxygen deprivation and nutrient deficiency during critical windows of brain growth can lead to lasting changes in brain structure and function. FGR has been linked to increased risk of cerebral palsy and to more subtle neurodevelopmental differences, including lower scores on cognitive and motor assessments in early childhood.²⁰

The evidence is particularly detailed in studies of monochorionic twins (identical twins sharing a placenta), where one twin is growth-restricted and the other serves as a built-in comparison. In these pairs, the smaller twin tends to score lower on developmental tests, sometimes by several points, and has a higher incidence of cerebral palsy compared with both the larger co-twin and twins from dichorionic pregnancies with similar weight differences.²¹ These within-pair comparisons are valuable because they control for genetics and family environment, strengthening the case that growth restriction itself drives part of the neurodevelopmental disadvantage.

FGR in Twin Pregnancies

Growth restriction in twins warrants separate mention because it involves unique biology. In monochorionic twins, the shared placenta contains blood vessel connections (anastomoses) between the two circulations. When the placental territory is divided unevenly, one twin may receive a substantially smaller share, leading to what is called selective fetal growth restriction, or sFGR.

Clinicians classify sFGR into three types based on the Doppler pattern in the smaller twin’s umbilical artery. Type I has a normal Doppler and generally carries the best outlook. Type II shows persistently absent or reversed blood flow, indicating severe placental insufficiency. Type III is the most unpredictable, with intermittent absent or reversed flow that can fluctuate from one scan to the next.²² Research into placental architecture suggests that the difference between types II and III may come down to the size of the artery-to-artery anastomosis connecting the twin circulations: a larger-diameter connection appears to create the intermittent pattern seen in type III by allowing blood pressure to fluctuate between the two sides.²³ Type III is particularly tricky to manage because the intermittent pattern can suddenly worsen, putting both twins at risk.

Long-Term Consequences for the Mother

FGR is often framed entirely around risks to the baby, but there is growing recognition that the mother’s health is affected too, both during and long after pregnancy. The cardiovascular strain of carrying a growth-restricted pregnancy, which often involves the same impaired blood-vessel remodeling that drives preeclampsia, can leave lasting changes in the mother’s heart and blood vessels. Studies have found persistent postpartum cardiovascular abnormalities in women who had FGR pregnancies, reinforcing the idea that pregnancy acts as a kind of stress test for the cardiovascular system.²⁴

A large Swedish study tracking mothers over years found that delivering a very small-for-gestational-age baby was associated with a meaningfully higher risk of later cardiovascular disease, even after adjusting for smoking, socioeconomic factors, and pregnancy complications. The risk was highest when the growth-restricted baby was also born preterm, with a hazard ratio of roughly 3.4 compared with mothers who delivered normally grown term infants.²⁵ This does not mean that FGR caused heart disease years later; it may be that the same underlying cardiovascular vulnerabilities that contributed to poor placental function also predispose these women to heart problems. Either way, the clinical takeaway is that women with a history of FGR may benefit from closer cardiovascular monitoring in the decades that follow.

The Emotional Weight of an FGR Diagnosis

Beyond the medical dimensions, an FGR diagnosis lands hard psychologically. Qualitative research exploring the experiences of pregnant women told they had FGR found high levels of anticipatory anxiety, including catastrophic thinking about developmental abnormalities, preterm birth, and the baby dying. The anxiety tended to intensify as the degree of growth restriction worsened, and the ongoing uncertainty between scans was a particular source of distress.²⁶ Women described feeling helpless because there was no treatment they could take to fix the problem. Clinicians who manage FGR pregnancies increasingly recognize that psychological support should be woven into the monitoring plan, not treated as an afterthought.

Socioeconomic and Ethnic Disparities

FGR does not distribute itself evenly across populations. In England, a national cohort study found that FGR rates roughly doubled between the least and most deprived socioeconomic groups, and about 31 percent of FGR cases could be attributed to socioeconomic inequality alone. Ethnic disparities were also stark: FGR rates ranged from about 1.4 percent in White women to 3.5 percent in South Asian women, with roughly 17 percent of FGR cases attributable to ethnic inequality.²⁷ The largest disparities were concentrated among Black and South Asian women living in the most deprived areas, where multiple disadvantages compound.

In the United States, research echoes these patterns but with some nuance. One study found that pregnant women facing social determinants of health challenges were about 16 percent more likely to experience FGR overall, though the effect varied by racial and ethnic group.²⁸ These disparities likely reflect a mix of factors: differences in access to prenatal care, chronic stress, nutritional quality, exposure to environmental pollutants, and the cumulative physiological toll of systemic disadvantage. Addressing FGR at a population level means addressing these upstream causes, not just refining clinical detection tools.

Epigenetic Changes and Generational Effects

One of the more unsettling findings in FGR research is that its effects may not stop with the affected individual. The metabolic reprogramming described earlier appears to involve epigenetic changes: chemical modifications to DNA and its packaging that alter how genes are expressed without changing the genetic code itself. In growth-restricted pregnancies, researchers have found altered patterns of DNA methylation and other epigenetic marks in the placenta and in the baby’s tissues, particularly in genes that govern growth, metabolism, and blood vessel development.²⁹

Animal studies have been especially revealing. In a rat model of growth restriction caused by protein deficiency during pregnancy, researchers identified specific gene expression changes in the placenta, including decreased activity of a gene called Wnt2 and increased activity of Dlk1, along with altered methylation patterns at specific sites in the Wnt2 gene.³⁰ Whether these epigenetic shifts can be transmitted across multiple generations in humans remains an open question. But the broader implication is provocative: a grandmother’s nutritional environment during pregnancy could theoretically influence her grandchild’s metabolic health. The human evidence for multigenerational transmission is still circumstantial, but the biological plausibility is strong enough that it is driving active research programs.