Fetal growth restriction, commonly abbreviated FGR, is a pregnancy complication in which a baby fails to reach its expected size in the womb because something has gone wrong with the supply of nutrients or oxygen. It affects roughly 5 to 10 percent of pregnancies worldwide, and it is one of the leading contributors to stillbirth, preterm delivery, and lasting health problems for the child. What makes FGR especially tricky is that a small baby is not always a sick baby, and telling the difference between a constitutionally small but healthy fetus and one that is genuinely struggling remains one of the harder problems in obstetric care.
Why FGR Happens
The most common driver of FGR is a poorly functioning placenta. In a healthy pregnancy, specialized cells called trophoblasts burrow deep into the walls of the uterus and remodel the mother’s spiral arteries so that blood flows freely to the placenta. When that remodeling is shallow or incomplete, the placenta never receives the blood supply it needs, and the fetus gradually falls behind in growth. This defective placentation is considered central to the development of both FGR and preeclampsia, particularly when either condition shows up before 32 weeks of pregnancy.1PubMed Central. Fetal Growth Restriction: Does an Integrated Maternal Hemodynamic-Placental Model Fit Better?
But the placenta is not always the sole culprit. Fetal causes include chromosomal abnormalities and congenital infections. Cytomegalovirus (CMV), one of the most common congenital infections globally, has been identified as an underlying cause of growth restriction even in pregnancies where the mother shows no symptoms.2PubMed Central. Intrauterine Growth Restriction Caused by Underlying Congenital Cytomegalovirus Infection Maternal conditions like chronic hypertension, kidney disease, autoimmune disorders, and diabetes also contribute. And then there are the modifiable risk factors, with smoking at the top of the list.
Smoking and Modifiable Risk Factors
Smoking during pregnancy has one of the strongest and most consistent associations with FGR of any behavioral factor. A large study using U.S. birth certificate data found that women who smoked throughout pregnancy had more than double the risk of delivering a growth-restricted baby compared with nonsmokers, even after adjusting for age, race, education, body weight, and coexisting conditions like hypertension and diabetes.3PubMed Central. Association of Reported Trimester-Specific Smoking Cessation and Fetal Growth Restriction The timing of quitting matters. Women who stopped smoking after just the first trimester still carried modestly elevated risk, but those who continued through the second trimester had roughly 70 to 80 percent higher odds of FGR, making early cessation far more protective than late cessation.
Secondhand smoke exposure also raises the risk, producing effects somewhere between those of active smoking and no exposure at all. Quitting by the end of the second trimester substantially reduces the chance of growth restriction and low birth weight, reinforcing the message that it is never too late to stop, though earlier is always better.4PubMed Central. Fetal growth restriction, low birth weight, and preterm birth: Effects of active or passive smoking evaluated by maternal expired CO at delivery, impacts of cessation at different trimesters
Nutritional deficiencies play a supporting role as well. Inadequate intake of folate, iron, zinc, and vitamin D has been linked to impaired fetal growth, and diets heavy in processed foods are associated with poorer pregnancy outcomes.5PubMed Central. Dietary interventions for fetal growth restriction – therapeutic potential of dietary nitrate supplementation in pregnancy Supplementation of these micronutrients is now standard practice in prenatal care across most developed countries, though deficiencies still slip through, particularly in populations with limited access to varied diets.
Early-Onset Versus Late-Onset FGR
One of the most important distinctions clinicians make is between early-onset FGR, typically diagnosed before 32 weeks of gestation, and late-onset FGR, diagnosed after 32 weeks. These are not simply mild and severe versions of the same problem. They tend to involve different patterns of placental damage and carry different risks.
Early-onset FGR is more strongly linked to severe problems with maternal blood supply to the placenta. Studies of placental tissue show that early-onset cases have far higher rates of vascular lesions from maternal underperfusion compared with appropriately grown control babies born at the same gestational age.6PubMed. The placental factor in early- and late-onset normotensive fetal growth restriction This pattern overlaps substantially with what is seen in severe early-onset preeclampsia, and indeed the two conditions share the same root problem of defective trophoblast invasion.7Fetal Diagnosis and Therapy. Placental Pathology in Early-Onset and Late-Onset Fetal Growth Restriction
Late-onset FGR, which accounts for the majority of cases, is a more mixed bag. These placentas still show increased villous lesions related to maternal underperfusion compared with healthy controls, but they also tend to have more fetal-side blood vessel problems, including clotting-related lesions. The combination of both maternal and fetal vascular compromise distinguishes late-onset FGR from the more purely maternal vascular story in early-onset disease.8PubMed. The placental factor in early- and late-onset normotensive fetal growth restriction This heterogeneity makes late-onset FGR harder to detect and harder to characterize on ultrasound, because the Doppler findings used to monitor blood flow are often subtler.
How Doctors Monitor a Growth-Restricted Fetus
Once FGR is suspected, the central question becomes how long the baby can safely stay in the womb versus how soon it needs to be delivered. Most of the surveillance toolkit revolves around Doppler ultrasound, which measures blood flow velocity in key fetal vessels.
The umbilical artery is typically the first vessel examined. In early-onset FGR, its Doppler readings are the most sensitive way to detect worsening placental resistance, picking up problems in about 70 percent of cases.9PubMed Central. Efficacy of Pulsatility Index of Fetal Vessels in Predicting Adverse Perinatal Outcomes in Fetuses with Growth Restriction – Differences in Early- and Late-Onset Fetal Growth Restriction As resistance in the umbilical artery climbs, blood flow during the relaxation phase of the heart cycle can diminish and eventually reverse, which is an ominous sign suggesting severe placental failure.
In late-onset FGR, the umbilical artery often looks deceptively normal. Instead, the ratio between blood flow in the fetal brain and the placenta becomes more informative. The cerebroplacental ratio and the middle cerebral artery were the most specific and diagnostically accurate measurements in late-onset disease in one study, correctly identifying adverse outcomes in close to 80 to 90 percent of cases.10PubMed Central. Efficacy of Pulsatility Index of Fetal Vessels in Predicting Adverse Perinatal Outcomes in Fetuses with Growth Restriction – Differences in Early- and Late-Onset Fetal Growth Restriction
When early-onset FGR progresses to its most severe stages, clinicians turn to the ductus venosus, a small shunt vessel in the fetal liver that carries oxygenated blood directly toward the heart. Abnormal flow patterns in the ductus venosus signal that the fetal heart is struggling to cope with the increased resistance, and an abnormal waveform in this vessel is generally considered a trigger for delivery regardless of gestational age.11PubMed Central. Harmony Behind the Trumped-Shaped Vessel: the Essential Role of the Ductus Venosus in Fetal Medicine Incorporating ductus venosus evaluation into routine surveillance helps clinicians balance the risks of prematurity from early delivery against the risk of worsening oxygen deprivation if delivery is delayed.12PubMed Central. Predictive Value of Venous Ductus Doppler in Perinatal Outcomes in Fetal Growth Restriction In general, worsening umbilical artery Doppler readings drive the clinical pace in early-onset FGR, while late changes in the ductus venosus or deterioration in biophysical measures like fetal movement or heart rate patterns can push the decision toward delivery.13PubMed. Planning management and delivery of the growth-restricted fetus
Brain Sparing and What It Actually Means
When a fetus is not getting enough oxygen, the body performs a kind of triage. Blood is preferentially redirected away from the gut, kidneys, and limbs toward the brain and heart, an adaptation called the “brain-sparing effect.” On Doppler ultrasound, this shows up as increased blood flow in the fetal brain arteries relative to the placental circulation. It sounds like a protective mechanism, and in the short term it is. But the long-term picture is less reassuring.
A systematic review found that brain sparing was associated with lower cognitive function and reduced IQ scores in childhood.14PubMed Central. Brain Sparing Effect on Neurodevelopment in Children with Intrauterine Growth Restriction: A Systematic Review The redistribution appeared linked to better executive function and behavior at age four, but that advantage had disappeared by age twelve. One early study demonstrated that the brain-sparing effect was most predictive of poor perinatal outcomes when it was detected before 34 weeks of gestation.15PubMed. The Doppler cerebroplacental ratio and perinatal outcome in intrauterine growth restriction
Research examining which specific brain arteries are affected has revealed that the pattern is not uniform. In one cohort, the anterior cerebral artery was the most frequently abnormal vessel, followed by the middle cerebral artery. When the vertebral artery showed abnormal readings, it was almost never the only vessel involved; most of those fetuses had abnormalities across two or more brain vessels, suggesting widespread redistribution rather than a focused adjustment.16PubMed Central. Patterns of Brain Sparing in a Fetal Growth Restriction Cohort
Long-Term Neurological Consequences
The brain does not emerge from growth restriction unscathed, even when the brain-sparing reflex has been doing its job. Children who experienced FGR tend to have smaller brain volumes, with reductions in cortical gray matter, the hippocampus, and the basal ganglia persisting well into childhood.17PubMed Central. Long-Term Neurologic Consequences following Fetal Growth Restriction: The Impact on Brain Reserve At age six, growth-restricted children showed weaker connectivity both within and between brain hemispheres. By age fifteen, reductions in cerebral cortex, white matter, and cerebellar cortex were still detectable.
There is a note of cautious optimism in the data. One analysis combining three cohorts at ages one, six, and ten found that the disrupted brain microstructure seen in FGR was slowly improving over time, suggesting a delay in brain maturation rather than permanent damage.18PubMed Central. Long-Term Neurologic Consequences following Fetal Growth Restriction: The Impact on Brain Reserve But a meta-analysis covering more than 50,000 children born preterm still found significant and persistent cognitive impairment at age twelve among those who had been growth-restricted. The clinical picture, then, is of brains that are trying to catch up but not always succeeding.
Late-onset FGR may carry a lighter neurological burden. A recent study following infants with late-onset FGR found no significant differences in cognitive, language, or motor development at 18 and 36 months compared with normally grown peers.19JAMA Network Open. Neurodevelopment Among Infants With Late-Onset Fetal Growth Restriction Gestational age at birth, rather than growth restriction itself, was the stronger predictor of early developmental scores, which further underscores why the timing of delivery in FGR is such a high-stakes decision.
Cardiovascular and Metabolic Risk in Adulthood
The consequences of FGR extend well beyond childhood. The “fetal origins hypothesis,” developed in the 1990s, proposed that undernutrition during mid to late gestation programs the body for metabolic and cardiovascular disease decades later. Research since then has confirmed that people whose birth weights were at the lower end of the normal range, or who were thin or short at birth, have increased rates of coronary heart disease.20PubMed Central. Fetal origins of coronary heart disease The proposed mechanisms include lasting changes to blood pressure regulation, insulin sensitivity, cholesterol metabolism, and hormonal settings.
More recent work using genetic methods has strengthened the case that this link is genuinely causal rather than just a statistical association. A study using Mendelian randomization found that lower birth weight, used as a proxy for restricted growth in the womb, is causally related to increased susceptibility to coronary artery disease and type 2 diabetes, and that this relationship is not explained by adult obesity or high blood pressure.21PubMed Central. Birthweight, Type 2 Diabetes Mellitus, and Cardiovascular Disease: Addressing the Barker Hypothesis With Mendelian Randomization In other words, the metabolic programming that occurs during restricted fetal growth appears to have an independent, direct effect on heart and metabolic health in adulthood.
Distinguishing True FGR From a Constitutionally Small Baby
Not every baby below the tenth percentile for weight is growth-restricted. Some are simply small because their parents are small, or because of normal ethnic or parity-related variation. Calling these babies “growth-restricted” leads to unnecessary interventions, while missing a truly growth-restricted baby who happens to fall above an arbitrary percentile cutoff can be dangerous.
Customized growth charts, which adjust expected fetal weight for maternal height, weight, ethnicity, and whether the mother has given birth before, may do a better job of sorting these groups apart. Large observational studies suggest that customized charts are better able to differentiate between constitutional and pathological smallness.22PubMed Central. Customised versus population-based growth charts as a screening tool for detecting small for gestational age infants in low-risk pregnant women However, reliable evidence from randomized trials comparing the two approaches is still lacking, so practice varies between hospitals and countries.
Blood-based biomarkers are emerging as a complementary tool. The ratio of two proteins in the mother’s blood, sFlt-1 and PlGF, reflects how well the placenta is functioning. A systematic review found that FGR pregnancies consistently showed higher sFlt-1/PlGF ratios than normally grown pregnancies, and that the ratio was especially discriminating in early-onset placental FGR.23PubMed. Maternal sFlt-1/PlGF Ratio to Distinguish Pathological Fetal Growth Restriction From Constitutional Smallness: Systematic Review Babies who were merely constitutionally small showed little or no elevation. A meta-analysis of eight studies estimated that an sFlt-1/PlGF ratio above a certain threshold had a pooled sensitivity of about 63 percent and specificity of about 84 percent for predicting FGR.24PubMed. Diagnostic capacity of sFlt-1/PlGF ratio in fetal growth restriction: A systematic review and meta-analysis
Combining ultrasound findings with this blood test appears to sharpen the prediction further. One study found that when ultrasound smallness was paired with an elevated sFlt-1/PlGF ratio at 28 weeks, the combination had a positive likelihood ratio above 40 for preterm delivery of a small baby, meaning a positive result dramatically raised the probability that something was genuinely wrong rather than constitutionally normal.25The Lancet. Screening for fetal growth restriction using fetal biometry combined with maternal biomarkers
FGR in Twin Pregnancies
Growth restriction takes on a different character in identical twins who share a placenta, known as monochorionic diamniotic twins. In these pregnancies, the two fetuses are connected by blood vessel anastomoses on the surface of their shared placenta, and if the placental territory is unequally divided, one twin can end up significantly smaller than the other. This condition, called selective fetal growth restriction, complicates roughly 10 to 15 percent of all monochorionic twin pregnancies.26PubMed Central. Selective Fetal Growth Restriction in Monochorionic Diamniotic Twins: Diagnosis and Management
Clinicians classify selective FGR in twins into three types based on the umbilical artery Doppler pattern of the smaller twin. In type I, blood flow in the umbilical artery looks normal and the outlook is generally favorable. In type II, there is persistently absent or reversed flow, which is associated with higher risk of fetal loss. Type III, characterized by intermittent absent or reversed flow, is the most unpredictable because the pattern can fluctuate. The presence and size of arterio-arterial anastomoses connecting the two sides of the placenta influence how stable the smaller twin’s circulation remains. When cord insertions of the two twins are close together, these connecting vessels tend to be larger, which may help buffer the smaller twin against sudden hemodynamic shifts.27PubMed. Proximate cord insertion in monochorionic twins with selective fetal growth restriction
Why There Is No Treatment Yet
The frustrating reality of FGR is that there is currently no way to fix it once it develops. There is no medication that can restore a poorly functioning placenta. The only “treatment” is managing the timing of delivery, weighing the risk of prematurity against the risk of leaving the baby in a hostile uterine environment.
One of the largest randomized trials of delivery timing in FGR, the Growth Restriction Intervention Trial, compared early delivery with delayed delivery in severely affected pregnancies. It highlighted the fundamental dilemma: both strategies carry real potential for harm, with one arm risking the complications of prematurity and the other risking prolonged oxygen deprivation.28American Journal of Obstetrics and Gynecology. The Growth Restriction Intervention Trial: long-term outcomes in a randomized trial of timing of delivery in fetal growth restriction
There was hope that sildenafil, a drug that relaxes blood vessels and is used to treat pulmonary hypertension in newborns, might improve blood flow to the placenta and buy time for severely growth-restricted babies. A multicenter trial in the United Kingdom tested this hypothesis. Sildenafil did not prolong pregnancy or improve outcomes in severe early-onset FGR, and the researchers concluded it should not be prescribed for this purpose outside of research settings.29The Lancet. Sildenafil as potential therapy for severely growth-restricted fetuses: an UK multicentre, randomised, placebo-controlled trial Other approaches, including maternal dietary supplementation with nitrate-rich foods, are under investigation, but none have yet moved the needle in clinical trials.
The Economic Weight of Severe Early-Onset FGR
Beyond the medical toll, severe early-onset FGR carries enormous financial costs, driven almost entirely by prolonged stays in the neonatal unit. A multicenter study in the United Kingdom calculated that the most premature infants born with early-onset FGR, those delivered before 28 weeks, cost roughly £158,000 more per baby than those delivered at term. Infants born between 28 and 32 weeks still cost about £94,000 more, and even moderately preterm infants born between 32 and 37 weeks incurred an extra £20,000 on average.30PubMed Central. Economic Impact of Severe Early-Onset Foetal Growth Restriction: A Multicentre Prospective Cohort Study These numbers, dominated by neonatal intensive care costs, make a strong economic case for investing in research that could delay delivery even by a few weeks, since each additional week in the womb at those early gestational ages translates to substantially lower healthcare spending and, far more importantly, substantially better outcomes for the child.
Epigenetic Changes in the Placenta
Researchers are increasingly looking at how calorie restriction and nutrient deprivation leave chemical marks on DNA in the placenta, changes that do not alter the genetic code itself but affect which genes get turned on or off. Animal studies have found that caloric restriction during pregnancy leads to a significant decrease in overall DNA methylation in the placenta, with the effects being more pronounced in males than in females.31PubMed Central. Intrauterine calorie restriction affects placental DNA methylation and gene expression Several of the gene pathways affected were already known to be associated with growth restriction. This sex-specific pattern is one reason researchers suspect that male and female fetuses may respond differently to the same nutritional insult, though translating these findings from mice to human clinical practice remains a long way off.