Intrauterine growth restriction (IUGR) is a condition in which a baby in the womb does not grow to its expected size for its gestational age. Clinically, it’s defined as an estimated fetal weight below the 10th percentile on ultrasound, meaning the baby is smaller than 90% of other babies at the same stage of pregnancy. IUGR affects roughly 8% of all pregnancies and is a significant contributor to complications during and after birth.
The terms “intrauterine growth restriction” and “fetal growth restriction” (FGR) are used interchangeably. Both describe a baby that isn’t reaching its genetic growth potential due to something going wrong with the mother’s health, the placenta, or the baby itself. Babies below the 3rd percentile face the highest risk of serious outcomes, including stillbirth, which is why many experts now use that stricter cutoff to identify the most severe cases.
Symmetric vs. Asymmetric Growth Restriction
Not all growth restriction looks the same. There are two main patterns, and they reflect different problems happening at different times in pregnancy.
Asymmetric IUGR is the more common type, accounting for 70% to 80% of all cases. It typically develops later in pregnancy when the placenta can’t deliver enough nutrients and oxygen. The baby’s body prioritizes blood flow to the brain, so the head grows normally while the belly and body fall behind. At birth, these babies often look thin and malnourished, with a noticeable size gap between head and chest circumference (more than 3 cm in a full-term baby). The good news: because brain development is largely preserved, the long-term outlook tends to be better.
Symmetric IUGR makes up the remaining 20% to 30% of cases. It starts earlier in pregnancy and affects everything proportionally: head, body, and limbs are all smaller than expected. This pattern points to something intrinsic to the baby, such as a chromosomal abnormality, a genetic syndrome, or a congenital infection. Because the insult begins during the earliest phases of cell division, the baby has fewer cells overall rather than just smaller cells. The prognosis is generally poorer, particularly when a genetic disorder is the underlying cause.
What Causes Growth Restriction
The causes fall into three broad categories: problems with the mother’s health, problems with the placenta, and problems originating in the baby.
Maternal Factors
High blood pressure, including preeclampsia, is one of the most common maternal drivers. Diabetes, cardiovascular disease, anemia, and significant malnutrition also restrict fetal growth by limiting the nutrients and oxygen available to the baby. Smoking and drug use during pregnancy directly impair blood flow to the placenta.
Less obvious risk factors have also been identified. Epilepsy during pregnancy increases the risk by about 28%, partly because certain anti-seizure medications cross the placenta and may affect fetal development. Hepatitis C infection raises the risk by roughly 53%. Celiac disease, structural uterine abnormalities, and thyroid conditions have all been linked to IUGR as well, though the evidence for these is not as strong. Depression during pregnancy also appears on the list of associated factors.
Placental Factors
The placenta is the baby’s lifeline, and when it underperforms, growth suffers. Placental insufficiency occurs when the small arteries in the placenta become damaged or blocked, increasing resistance to blood flow and reducing the delivery of oxygen and amino acids to the baby. Placental abruption (where the placenta partially separates from the uterine wall) roughly doubles the risk of IUGR. Placental infarcts, where sections of tissue die off, have a similar effect.
Even placenta previa, where the placenta sits unusually low in the uterus, carries a modestly elevated risk. In many cases, no specific cause is found, and the placental insufficiency is labeled idiopathic.
Fetal Factors
Chromosomal abnormalities and genetic syndromes are the primary fetal causes. Mutations in genes that regulate key growth-signaling hormones can cause both prenatal and postnatal growth restriction. Congenital infections, particularly the TORCH group (toxoplasmosis, rubella, cytomegalovirus, herpes), damage developing tissues early and tend to produce the symmetric pattern of restricted growth. Carrying multiples also increases the risk, as the babies compete for a shared nutrient supply.
How IUGR Is Diagnosed
Diagnosis starts with a routine ultrasound that measures the baby’s estimated weight and compares it to standard growth charts for that gestational age. If the weight falls below the 10th percentile, growth restriction is suspected. A measurement of the baby’s abdominal circumference is particularly telling: in asymmetric IUGR, it’s often the first measurement to drop because the liver, which stores energy, shrinks before the head does.
Once suspected, Doppler ultrasound becomes the key monitoring tool. This measures blood flow through the umbilical artery, which connects the baby to the placenta. In a healthy pregnancy, resistance in the umbilical artery decreases steadily as the placenta matures. In IUGR, that resistance stays elevated or worsens. In severe cases, blood flow during the resting phase of each heartbeat can disappear entirely (absent end-diastolic flow) or even reverse direction. These findings signal serious placental dysfunction and often trigger decisions about early delivery.
Doctors also measure blood flow in the baby’s middle cerebral artery. When the brain senses it isn’t getting enough oxygen, it dilates its blood vessels to pull in more, a compensatory response called “brain sparing.” The ratio between umbilical and cerebral blood flow helps distinguish a baby that is simply small from one that is genuinely growth-restricted and under stress.
Monitoring During Pregnancy
How closely you’re monitored depends on when the restriction is detected and how severe it is. Early-onset IUGR, diagnosed before 32 weeks, is managed at specialized maternal-fetal medicine centers. When the umbilical artery shows absent or reversed blood flow, surveillance intensifies to every two to three days, with assessments of fetal heart rate patterns and biophysical scoring to watch for signs of distress.
Late-onset IUGR, which appears after 32 weeks, is typically monitored with weekly or twice-weekly Doppler assessments. If growth restriction is milder, with the baby measuring small but blood flow still looking relatively normal, fetal growth is re-evaluated every two weeks with weekly Doppler checks in between. The goal of all this monitoring is to balance two competing risks: delivering too early versus leaving a struggling baby in a hostile environment too long.
Complications at Birth
Babies born with growth restriction face a cluster of immediate challenges. Their limited fat stores make it hard to regulate body temperature, so they’re prone to losing heat quickly after birth. Low blood sugar is common because they have minimal energy reserves, and it can develop within hours of delivery. Other frequent complications include elevated bilirubin (jaundice), difficulty feeding, a higher-than-normal red blood cell count that thickens the blood, low calcium levels, and a weakened immune response.
Many growth-restricted babies are also born preterm, either because labor begins spontaneously or because doctors determine that delivering early is safer than continuing the pregnancy. Prematurity compounds every other risk, adding breathing difficulties and a longer stay in the neonatal intensive care unit.
Long-Term Health Consequences
The effects of growth restriction don’t end at birth. Decades of research, grounded in what’s known as the developmental origins of disease, have established that babies who were growth-restricted carry elevated health risks well into adulthood.
Cardiovascular disease is the most studied long-term outcome. A landmark study following over 15,000 births in Sweden found a significant link between low birth weight and death from heart disease in men aged 65 and older, even after accounting for socioeconomic differences. Another study of 50-year-olds in the UK found that systolic blood pressure rose by 2.7 mmHg for every 450-gram decrease in birth weight. The Cardiovascular Risk in Young Finns study tracked nearly 3,600 people from childhood into middle age and found that those affected by IUGR showed subtle changes in heart structure and reduced pumping efficiency.
Metabolic problems are equally concerning. Growth-restricted babies who then experience rapid “catch-up” growth after birth appear to be at higher risk for obesity, insulin resistance, type 2 diabetes, and abnormal cholesterol levels later in life. The current theory is that a baby deprived of nutrients in the womb programs its metabolism for scarcity. When it’s born into a world of adequate or excess nutrition, that mismatch promotes metabolic disease.
Neurodevelopmental outcomes vary depending on the type and severity. Symmetric IUGR, which affects brain growth from early pregnancy, tends to carry a higher risk of cognitive and developmental difficulties. Asymmetric IUGR, where the brain is relatively spared, generally has better neurological outcomes, though subtle differences in learning, attention, and motor skills have been documented in follow-up studies.
Prevention Options
Prevention is limited because many causes of IUGR, such as genetic factors and unexplained placental failure, aren’t modifiable. The most effective strategies focus on managing known risk factors: controlling blood pressure, treating underlying conditions, ensuring adequate nutrition, and avoiding smoking and substance use during pregnancy.
Low-dose aspirin (81 mg daily), started before 16 weeks of pregnancy, is recommended for people at high risk of preeclampsia, which is itself a major driver of growth restriction. However, the American College of Obstetricians and Gynecologists does not recommend aspirin specifically for preventing IUGR in the absence of preeclampsia risk factors. For people with conditions like epilepsy, careful medication management before and during pregnancy can help reduce the chance of fetal growth being affected.