Intrauterine growth retardation (IUGR), now more commonly called fetal growth restriction (FGR), is a pregnancy condition in which a baby fails to reach its expected size in the womb. It’s defined as an estimated fetal weight or abdominal circumference below the 10th percentile for gestational age on ultrasound. The condition affects roughly one in five babies in low- and middle-income countries, with the highest rates (34%) in South Asia. In populations with optimal prenatal nutrition and health, about 10% of infants are born smaller than expected, though not all of those cases involve a true growth problem.
Growth Restriction vs. Small for Gestational Age
You’ll often see the terms “small for gestational age” (SGA) and “fetal growth restriction” used interchangeably, but they mean different things. SGA simply describes a baby whose size falls below the 10th percentile at birth. Some of these babies are just constitutionally small, perfectly healthy but genetically programmed to be petite. Fetal growth restriction, on the other hand, means the baby isn’t growing as it should because something is going wrong, most often with the placenta.
When an ultrasound shows a baby measuring below the 10th percentile, that finding should trigger further evaluation to determine whether there’s an underlying problem or whether the baby is simply small. The risk of serious complications rises steeply for babies below the 3rd percentile, and many experts now use that stricter cutoff to identify the most vulnerable cases.
Two Patterns of Restricted Growth
Growth restriction generally follows one of two patterns, and the distinction matters because each points to different causes and carries different implications.
Symmetric (proportional) growth restriction typically begins early in pregnancy. The baby’s head, abdomen, and limbs are all uniformly small. This pattern tends to result from problems intrinsic to the baby itself: chromosomal abnormalities, genetic syndromes, or infections contracted early in pregnancy (particularly the TORCH group, which includes toxoplasmosis, rubella, cytomegalovirus, and herpes). In symmetric growth restriction, the total number of cells in the baby’s body is reduced, meaning every organ is affected from the start. These babies may also show signs like unusual facial features, congenital heart defects, or an enlarged liver and spleen.
Asymmetric (disproportionate) growth restriction develops later in pregnancy and is far more common. Here, the baby’s head and limbs measure normally while the abdomen is small. This happens because when the placenta can’t deliver enough nutrients, the baby’s body redirects blood flow to protect the brain and heart at the expense of the liver and other abdominal organs. Cell size shrinks, but cell number stays normal. The primary cause is placental insufficiency, meaning the placenta isn’t functioning well enough to keep up with the baby’s growing demands. This “brain-sparing” pattern generally carries a better outlook because the brain is relatively protected.
What Causes It
The most common cause of fetal growth restriction is placental dysfunction. The placenta is the baby’s sole source of oxygen and nutrients, and when it underperforms, growth slows. Conditions that damage blood flow to the placenta, particularly preeclampsia and chronic high blood pressure, are major drivers. Other maternal factors include smoking, heavy alcohol use, drug exposure, severe malnutrition, and chronic conditions like diabetes or kidney disease.
Fetal causes include chromosomal abnormalities (such as trisomy 13 or 18), structural birth defects, and congenital infections. Up to 20% of cases diagnosed before 32 weeks of gestation are linked to chromosomal or structural abnormalities, which is why early-onset growth restriction prompts genetic testing. Multiple pregnancies (twins, triplets) also raise the risk, since the placenta must support more than one baby.
How It’s Detected
During routine prenatal visits, your provider measures the distance from your pubic bone to the top of the uterus, called the fundal height. This simple tape-measure check catches between 56% and 86% of small babies, depending on technique. It’s a useful screening tool but far from perfect, which is why an ultrasound follows any time the measurement seems off.
Ultrasound is the primary diagnostic tool, with a detection rate for growth restriction as high as 93%. It estimates the baby’s weight by combining measurements of the head, abdomen, and thigh bone. When growth restriction is suspected, Doppler ultrasound becomes critical. This measures blood flow through the umbilical cord and other key vessels, providing a window into how well the placenta is functioning. Abnormal flow patterns in the umbilical artery, such as absent or reversed blood flow at the end of each heartbeat, signal serious placental resistance and indicate the baby may be in distress.
Early-onset growth restriction (before 32 weeks) and late-onset growth restriction (32 weeks and beyond) are evaluated somewhat differently. In early-onset cases, the key markers are an abdominal circumference below the 3rd percentile and abnormal umbilical artery Doppler readings. Early-onset cases show abnormal umbilical artery Doppler roughly twice as often as late-onset cases (57% vs. 28%), reflecting more severe placental problems.
Monitoring and Delivery Decisions
There is no treatment that can reverse growth restriction once it’s established. Bed rest and medications like sildenafil have been studied and are not recommended, as they haven’t been shown to help. Instead, management centers on close surveillance of the baby and deciding the right time to deliver.
Once growth restriction is identified, umbilical artery Doppler monitoring begins and continues at regular intervals. Providers also use biophysical profiles, which combine ultrasound observation of fetal movement, breathing, and muscle tone with amniotic fluid levels to assess the baby’s well-being. The goal is to balance two competing risks: the dangers of prematurity if the baby is delivered too early versus the dangers of leaving a struggling baby in an environment that can no longer support it.
For early-onset growth restriction (between 26 and 32 weeks), delivery is typically triggered by specific warning signs: reversed blood flow in the ductus venosus (a critical vessel near the baby’s heart), an abnormal biophysical profile score, or concerning heart rate patterns. For later-onset cases, the thresholds are somewhat different because the baby is more mature and better equipped to handle delivery. Severe complications in the mother, like eclampsia or HELLP syndrome, override all other considerations and require immediate delivery regardless of gestational age.
Long-Term Health Effects
The consequences of growth restriction don’t end at birth. Babies born with this condition face a higher risk of complications in the newborn period, including breathing difficulties, low blood sugar, and trouble regulating body temperature. Many undergo a period of “catch-up growth” in the first years of life, but this rapid postnatal growth may itself carry risks.
Research following growth-restricted babies into adulthood has revealed a consistent pattern: low birth weight programs the body for chronic disease later in life. Adults who were growth-restricted in the womb have higher rates of high blood pressure, coronary artery disease, stroke, heart failure, type 2 diabetes, obesity, and insulin resistance. A UK study of 50-year-olds found that for every pound less a person weighed at birth, their systolic blood pressure was measurably higher in middle age. A Swedish study of over 15,000 births found a significant link between low birth weight and death from heart disease in men over 65, even after accounting for socioeconomic differences.
The Cardiovascular Risk in Young Finns study, which tracked over 3,500 people from childhood into their 30s and 40s, found that those affected by growth restriction showed subtle changes in heart structure and reduced pumping efficiency as adults. Neurodevelopmental effects are also documented, with some growth-restricted children showing difficulties with learning, attention, and motor skills that persist into school age. The mechanisms behind these long-term effects involve permanent changes in organ structure and metabolic programming that occur during fetal development, essentially setting the body’s baseline responses to stress, blood sugar regulation, and cardiovascular function at a disadvantage from the start.