The most common reason a baby stops growing properly in the womb is that the placenta fails to deliver enough oxygen and nutrients. This condition, called fetal growth restriction (FGR), affects a meaningful fraction of pregnancies and can stem from problems with the mother’s health, the placenta’s structure, the baby’s genetics, infections, or environmental exposures. The underlying cause matters because it shapes how severe the restriction is, when it appears during pregnancy, and what can be done about it.
The Placenta as the Central Player
In most cases of fetal growth restriction, the problem traces back to the placenta. Early in pregnancy, specialized cells from the embryo invade the walls of the mother’s uterine arteries, widening them so they can carry the large volumes of blood the growing baby will need. When this remodeling fails, the arteries stay narrow, blood flow to the placenta is restricted, and the baby receives less oxygen and fewer nutrients than it needs to grow on schedule. This failure of spiral artery remodeling is considered the root cause of both fetal growth restriction and preeclampsia, and the two conditions frequently overlap.1PubMed. Spiral artery remodeling and trophoblast invasion in preeclampsia and fetal growth restriction: relationship to clinical outcome
The tricky part is that a small baby is not necessarily a sick baby. Some fetuses are just constitutionally small, growing along a lower percentile because of their parents’ size or ethnic background, but developing normally. Telling these babies apart from those experiencing genuine placental insufficiency is one of the harder problems in prenatal care, especially when the estimated weight falls between the 3rd and 10th percentiles. Standard blood-flow measurements through the umbilical cord can look normal until placental dysfunction becomes quite advanced.2PubMed Central. Assessment of fetal cardiac function using the modified myocardial performance index in fetuses with growth restriction: a prospective observational study
Maternal Health Conditions That Restrict Growth
Preeclampsia is one of the best-studied maternal conditions linked to poor fetal growth. It involves high blood pressure and organ damage during pregnancy, and it directly impairs placental blood flow. When preeclampsia is mild, the reduction in birth weight is modest. But in severe or early-onset cases, the impact is dramatic: early-onset preeclampsia has been associated with birth weights roughly a quarter lower than expected, and the risk of delivering a baby that is small for gestational age is about four times higher than in uncomplicated pregnancies.3PubMed. Preeclampsia and fetal growth When preeclampsia develops in the second trimester and progresses to HELLP syndrome, a serious complication involving liver damage and low blood-clotting cells, the resulting placental insufficiency can be severe enough to restrict growth very early.4PubMed Central. Early Onset Preeclampsia and Intrauterine Growth Restriction: A Case Report
Anemia during pregnancy is another culprit, and it is far more common worldwide than preeclampsia. When a mother’s hemoglobin drops too low, her blood carries less oxygen and fewer nutrients to the placenta. The body tries to compensate by growing new blood vessels in the placenta, but this remodeling is often inadequate, and the fetus ends up chronically undersupplied.5PubMed Central. Complications of anemia in pregnancy: An updated overview for healthcare professionals Other chronic maternal conditions that impair blood flow or oxygen delivery, including poorly controlled diabetes, kidney disease, and autoimmune disorders like lupus, carry similar risks. The common thread is always the same: something on the mother’s side reduces the supply of what the placenta needs to do its job.
Infections That Cross to the Baby
Certain infections can directly damage the baby or the placenta. Cytomegalovirus (CMV) is one of the more important ones. It is a common virus that most adults carry without symptoms, but when a mother is infected for the first time during pregnancy, or when a dormant infection reactivates, the virus can cross the placenta and infect the fetus. Congenital CMV can cause fetal growth restriction along with other serious problems, including hearing loss and brain abnormalities.6PubMed Central. Fetal Ultrasound and Magnetic Resonance Imaging Abnormalities in Congenital Cytomegalovirus Infection Associated with and without Fetal Growth Restriction
CMV is not the only infection that matters. Toxoplasmosis, rubella, syphilis, and Zika virus can all restrict fetal growth through similar mechanisms: either by infecting placental tissue and impairing its function, or by infecting the fetus directly and disrupting normal organ development. These infections are grouped under the clinical shorthand “TORCH” infections. The growth restriction they cause tends to be symmetric, meaning the baby’s head, abdomen, and limbs are all proportionally small, because the damage typically begins early in development and affects the baby’s cells broadly.
Genetic and Chromosomal Causes
Sometimes the baby’s own genes are responsible. Chromosomal abnormalities, such as trisomy 18 or trisomy 21, are well-known causes of growth restriction. But the genetic picture is broader than just whole-chromosome problems. Smaller-scale variations in DNA, called copy number variants, can also restrict growth. In one study of 151 growth-restricted fetuses who underwent genetic testing, about one in eight had a chromosomal abnormality detected, including several cases of pathogenic copy number variants that would not have been caught by traditional chromosome analysis alone.7PubMed. The genetics and clinical outcomes in 151 cases of fetal growth restriction: A Chinese single-center study
Beyond chromosomes, single-gene disorders, epigenetic changes, and a phenomenon called confined placental mosaicism can all play a role.8PubMed Central. Genetics Etiologies Associated with Fetal Growth Restriction In confined placental mosaicism, the placenta carries a genetic abnormality that the baby itself does not. The abnormal cells in the placenta don’t function properly, so the organ underperforms even though the fetus is genetically normal. This is one reason that growth restriction sometimes appears without an obvious maternal cause: the problem is hiding in the placenta’s own DNA. Genetic factors are critical enough that current practice increasingly includes chromosomal microarray testing when growth restriction is found and no other explanation is apparent.9PubMed Central. Genetic Background of Fetal Growth Restriction
Smoking, Air Pollution, and Other Environmental Exposures
Maternal smoking is one of the most preventable causes of fetal growth restriction. The mechanism goes beyond simply reducing oxygen: carbon monoxide from cigarette smoke actively suppresses signaling pathways that placental cells need to grow and divide. Research has shown that carbon monoxide exposure leads to reduced cellular growth signaling in placental cells, and in animal models, exposing pregnant mice to carbon monoxide analogs in late pregnancy produced fetal growth restriction.10PubMed Central. Excess placental secreted frizzled-related protein 1 in maternal smokers impairs fetal growth The effect is dose-dependent: heavier smoking causes more restriction, and quitting, even partway through pregnancy, reduces the risk.
Ambient air pollution appears to work through similar pathways. Exposure to pollutants like nitrogen dioxide has been linked to reduced blood-vessel density in the placenta, which limits how much blood reaches the fetus. Air pollution also increases oxidative stress, which damages the energy-producing structures inside placental cells and further reduces the organ’s capacity to transfer nutrients.11PubMed Central. Ambient air pollution and fetal growth restriction: physician diagnosis of fetal growth restriction versus population-based small-for-gestational age Alcohol, certain drugs, and malnutrition round out the list of avoidable environmental exposures. The practical takeaway is that many of the environmental causes of growth restriction are modifiable, and addressing them early in pregnancy offers a real chance to improve fetal growth.
Umbilical Cord and Placental Structural Problems
Where the umbilical cord attaches to the placenta matters more than most people realize. Normally, the cord inserts near the center of the placenta, giving the blood vessels direct and efficient access to the tissue. In a condition called velamentous cord insertion, the cord attaches at the edge of the placenta, and the blood vessels must travel unprotected across the membranes before reaching placental tissue. This arrangement increases vascular resistance and hampers nutrient transfer.12PubMed Central. Prevalence, Risk Factors and Outcomes of Velamentous and Marginal Cord Insertions: A Population-Based Study of 634,741 Pregnancies
Systematic reviews have found that velamentous cord insertion roughly doubles the risk of delivering a baby that is small for gestational age.13PubMed. Impact of velamentous cord insertion on perinatal outcomes: a systematic review and meta-analysis The condition also carries a substantially higher risk of intrauterine fetal death.14PubMed Central. A systematic review and meta-analysis of velamentous cord insertion among singleton pregnancies and the risk of preterm delivery This is a structural cause of growth restriction that is nobody’s fault and cannot be prevented. It can, however, be detected on ultrasound, which is one reason thorough ultrasound assessment matters.
Twin Pregnancies and Unequal Sharing
Growth restriction takes on a different character in twin pregnancies, especially in identical twins who share a placenta. In these monochorionic twins, the placenta’s blood-vessel network is shared, and the division is not always equal. Selective fetal growth restriction, where one twin grows normally while the other falls behind, complicates roughly 10% to 15% of shared-placenta twin pregnancies.15PubMed Central. Selective Fetal Growth Restriction in Monochorionic Diamniotic Twins: Diagnosis and Management The severity depends on the pattern of blood flow in the smaller twin’s umbilical artery, which clinicians classify into three types based on Doppler readings. In the mildest form, blood flow in the smaller twin’s umbilical artery remains normal and outcomes are generally good. In more severe types, flow patterns become abnormal or reverse direction, carrying a higher risk of the smaller twin dying in the womb.
Managing selective growth restriction in twins is particularly challenging because any intervention that affects the placenta’s blood-vessel connections could harm the healthy twin. In the most severe cases, options range from very close monitoring to laser treatment that separates the shared blood-vessel connections, effectively giving each twin an independent blood supply.
How the Baby Adapts: Brain Sparing
When oxygen supply drops, the fetus does not just passively shrink. It actively redistributes its blood flow to protect the most critical organs, especially the brain. This adaptive response, called brain sparing, involves constricting blood vessels in the limbs and abdomen to redirect blood toward the head. It is an elegant survival strategy in the short term, but it comes at a cost.16PubMed Central. Brain sparing in fetal growth restriction: The double-edged sword of fetal hypoxaemia
The organs that lose blood supply during brain sparing, particularly the gut, kidneys, and liver, may develop differently as a result. And brain sparing itself is not without neurological risk. Research increasingly shows that even when the brain receives preferential blood flow, chronic oxygen deprivation still affects brain development. The redistribution buys time, but if the growth restriction is not recognized and managed, the advantage is limited. On ultrasound, brain sparing shows up as increased blood flow in the brain’s middle cerebral artery relative to the umbilical artery, and it is one of the signs doctors look for when monitoring a growth-restricted baby.
How Growth Restriction Is Detected and Monitored
The primary tool for detecting fetal growth restriction is ultrasound. Measurements of the baby’s head, abdomen, and femur are used to estimate fetal weight, and when that weight falls below the 10th percentile for gestational age, growth restriction is suspected. But a single measurement is not always diagnostic. Serial ultrasounds that show a growth curve flattening or crossing downward are more informative than a single low reading.
Once growth restriction is suspected, Doppler ultrasound of the umbilical artery becomes the main monitoring tool. In high-risk pregnancies where growth restriction is suspected, using umbilical artery Doppler to guide management has been shown to reduce the need for labor induction and cesarean delivery and to lower perinatal deaths compared to pregnancies managed without Doppler.17American Journal of Obstetrics and Gynecology. Doppler assessment of the fetus with intrauterine growth restriction Doppler studies of other blood vessels, including the ductus venosus and the middle cerebral artery, provide additional information about how well the baby is coping. They can reveal whether brain sparing is occurring or whether the baby’s heart is beginning to strain. However, the evidence base for using those additional vessels to guide clinical decisions outside of research settings is still developing.
When to Deliver
There is no treatment that reverses fetal growth restriction once it is established. The central management question is about timing: at what point does the risk of keeping the baby inside outweigh the risk of delivering early? Too-early delivery means prematurity, with all its complications. Too-late delivery risks stillbirth or permanent organ damage from chronic oxygen deprivation.
This decision hinges on the gestational age and the severity of the abnormalities seen on fetal monitoring, balancing prematurity risks against the risk of serious harm or death if the pregnancy continues.18PubMed Central. Fetal Growth Restriction: Contemporary Evidence to Guide Delivery Timing and Intrapartum Management In mild cases with normal Doppler readings, expectant management with frequent monitoring can safely continue until close to full term. In severe early-onset cases where Doppler shows absent or reversed blood flow in the umbilical artery, delivery may need to happen much earlier, sometimes before 32 weeks, because the risk of the baby dying in the womb becomes unacceptably high. Corticosteroids are given to accelerate the baby’s lung development before early delivery. The decisions involved are some of the most difficult in obstetrics, balancing competing risks with imperfect information.
Long-Term Health Consequences for the Child
The effects of fetal growth restriction do not end at birth. Research into what is often called fetal programming suggests that the conditions a baby experiences in the womb leave lasting marks on metabolism and cardiovascular health. Low birth weight resulting from poor fetal growth has been linked to a higher risk of coronary artery disease, high blood pressure, obesity, and insulin resistance later in life.19PubMed Central. Fetal origins of adult disease
Paradoxically, how these babies are fed after birth also matters. When a growth-restricted baby gains weight very rapidly after delivery, a phenomenon called catch-up growth, the metabolic benefits of improved nutrition can be offset by an increased risk of metabolic syndrome later, including glucose intolerance and abnormal cholesterol levels. This creates a difficult balancing act for pediatricians: the infant needs to grow, but excessively rapid weight gain carries its own long-term risks. The current understanding is that moderate, well-paced nutritional support is preferable to aggressive caloric loading in the weeks and months after delivery.
The Evolutionary Tug-of-War Behind Fetal Growth
There is a fascinating evolutionary dimension to fetal growth that sheds light on why placental function is so delicately balanced. In mammals, genes inherited from the father tend to promote fetal growth and resource extraction from the mother, while genes inherited from the mother tend to restrain it. This is not a metaphor; specific genes controlling placental development are physically silenced depending on which parent they came from, a process called genomic imprinting.20PubMed. The conflict theory of genomic imprinting: how much can be explained?
The leading explanation for this is the genetic conflict hypothesis. Because the father’s genes benefit from extracting as many resources as possible from this particular mother (since the father may have offspring with other mothers), paternally expressed genes tend to push for more growth. Meanwhile, the mother’s genes benefit from distributing her resources across all her offspring, current and future, so maternally expressed genes tend to restrain growth.21Population Ecology. Conflict theory of genomic imprinting in mammals When the balance between these opposing genetic forces is disrupted, through mutations, epigenetic errors, or conditions like confined placental mosaicism, abnormal growth can result. These imprinted genes are directly involved in controlling fetal size, placental development, and maternal-fetal nutrient exchange.22PubMed. Genetic conflict in early development: parental imprinting in normal and abnormal growth
Emerging Treatments Targeting the Placenta
Because fetal growth restriction cannot currently be reversed once diagnosed, researchers are working on treatments that could improve placental blood flow without affecting the rest of the mother’s body. One of the more promising experimental approaches uses short protein chains called homing peptides that can deliver drugs specifically to placental blood vessels. A peptide sequence called NKGLRNK has been shown to deliver a blood-vessel-widening drug directly to the uterine-placental blood supply in both mice and human placental tissue, without accumulating in any other maternal or fetal tissues.23PubMed Central. Placenta-targeted Treatment Strategies for Preeclampsia and Fetal Growth Restriction: An Opportunity and Major Challenge This kind of targeted delivery is exciting because the main barrier to treating placental insufficiency has always been the risk that systemic drugs would affect the mother or the baby in unintended ways.
Sildenafil, the drug better known by its commercial name for treating erectile dysfunction, has also been studied in growth-restricted pregnancies. It works by widening blood vessels, and early research suggested it could improve blood flow through the uterine arteries and increase fetal weight.24International Journal of Reproduction, Contraception, Obstetrics and Gynecology. Sildenafil citrate and uteroplacental perfusion in fetal growth restriction However, a large clinical trial (the STRIDER trial) was stopped early after raising safety concerns, and sildenafil is not recommended for this purpose outside of research settings. The episode illustrates both the urgency of finding treatments and the difficulty of intervening in a system as finely balanced as the placenta.