IUGR, or intrauterine growth restriction, is diagnosed when a baby in the womb measures below the 10th percentile for its gestational age on ultrasound. That means the baby is smaller than 90% of other babies at the same stage of pregnancy. The measurement can be based on either the estimated fetal weight or the abdominal circumference. Medical organizations now prefer the term “fetal growth restriction” (FGR), but IUGR remains widely used and refers to the same condition.
Being small doesn’t always mean something is wrong. Some babies are naturally petite. Growth restriction becomes a concern when the small size is caused by something preventing the baby from reaching its genetic growth potential, most commonly a problem with the placenta’s ability to deliver nutrients and oxygen.
How Growth Restriction Is Classified
There are two patterns of restricted growth, and they look different on ultrasound because they affect the baby’s body in different ways.
Symmetric growth restriction means the entire body is proportionally small, including the head. Both the estimated fetal weight and the head measurement (biparietal diameter) fall below the 10th percentile. This pattern tends to start earlier in pregnancy and can point to a genetic condition, a chromosomal abnormality, or an infection that affected the baby during a critical period of organ development.
Asymmetric growth restriction means the baby’s head continues to grow normally while the body falls behind. On ultrasound, the estimated weight is below the 10th percentile but the head measurement stays above it. This happens because the baby’s circulatory system prioritizes blood flow to the brain at the expense of the abdomen and limbs. Asymmetric restriction is the more common pattern and is usually tied to placental problems that develop later in the second or third trimester.
What Causes It
The most frequent cause is placental insufficiency, where the placenta can’t move enough oxygen and nutrients to the baby. This can happen because of physical damage to the placenta itself: areas of tissue death (infarcts), heavy fibrin deposits that block blood exchange, or narrowing of the spiral arteries that feed the placenta from the mother’s side. Abnormalities in the umbilical cord or unusual placental shape can also limit how well nutrients transfer.
At a genetic level, certain genes that regulate placental growth can be thrown off balance. In growth-restricted pregnancies, genes that slow placental development tend to be overactive while genes that promote growth are underactive.
Several maternal health conditions raise the risk:
- High blood pressure, preeclampsia, or heart disease
- Diabetes
- Autoimmune conditions such as lupus
- Kidney disease
- Anemia or sickle cell disease
- Infections including rubella, syphilis, CMV, and toxoplasmosis
- Carrying twins or triplets
- A previous pregnancy with growth restriction
Smoking, alcohol use, and recreational drug use are the main modifiable risk factors. Certain anti-seizure medications can also contribute.
How Doctors Detect and Monitor It
Growth restriction is typically caught during a routine ultrasound when the baby measures smaller than expected. Once it’s suspected, Doppler ultrasound becomes the key tool for assessing how serious the problem is. Doppler measures blood flow through specific vessels, and the patterns tell doctors a great deal about how the placenta and baby are coping.
The umbilical artery is the most important vessel to monitor. In a healthy pregnancy, blood flows steadily through it in both directions. When placental disease destroys the tiny arteries inside the placenta, blood flow during the resting phase of each heartbeat (diastole) gradually decreases. If more than about 70% of those small placental arteries are blocked, the diastolic flow can disappear entirely or even reverse direction. Absent or reversed end-diastolic flow is a sign of severe compromise and is strongly associated with very low birth weight (below the 3rd percentile), low amniotic fluid, stillbirth, and neurological injury.
Doctors also sometimes check blood flow through the ductus venosus, a small vessel near the baby’s liver that reflects how well the heart is handling the strain. Absent or reversed flow in this vessel during late diastole is associated with fetal acidemia (a dangerous drop in blood pH) and a higher risk of perinatal death. In one study of 121 growth-restricted pregnancies, stillbirth only occurred when this vessel showed reversed flow.
Monitoring Frequency
How often you’ll have monitoring depends on the Doppler results. If the umbilical artery shows some reduced flow but diastolic flow is still present, once or twice weekly monitoring is typical. When diastolic flow is absent, or when there’s also low amniotic fluid or a maternal complication, twice-weekly monitoring or even inpatient observation may be recommended. Reversed end-diastolic flow generally calls for inpatient management with close surveillance.
When Delivery Is Recommended
Timing delivery is a balancing act between the risks of keeping a compromised baby in the womb and the risks of premature birth. The severity of the restriction and the Doppler findings guide that decision.
For milder cases where the baby’s weight falls between the 3rd and 10th percentile and all Doppler measurements look normal, delivery is recommended between 40 and 41 weeks. If there are subtle signs of concern, like a significant drop in abdominal circumference or borderline blood flow ratios, that window moves to 39 to 40 weeks. For high-risk cases, including babies below the 3rd percentile, abnormal blood flow in the uterine arteries, or elevated resistance in the umbilical artery, delivery is recommended between 37 and 38 weeks. Babies with absent or reversed end-diastolic flow may need to be delivered even earlier, depending on how they respond to monitoring.
Long-Term Effects on the Baby
Most babies born with mild growth restriction catch up in size and develop normally. The concern grows with severity, especially when growth restriction is combined with preterm birth.
Research comparing very preterm babies born with growth restriction to preterm babies of normal size has found meaningful differences in brain development. Growth-restricted babies showed structural changes in brain gray matter, particularly in regions involved in emotion, memory, and sensory processing. By toddlerhood, these children scored lower on cognitive and motor assessments, with moderate effect sizes of around 0.4 on standardized scales. They were also roughly twice as likely to screen positive on an autism screening checklist. The takeaway from this research is that growth restriction appears to carry neurodevelopmental risks above and beyond those of prematurity alone.
Beyond brain development, babies born with growth restriction face a higher lifetime risk of metabolic and cardiovascular problems. The theory, well supported by decades of research, is that a nutrient-deprived fetus permanently reprograms its metabolism to store calories efficiently. That adaptation becomes a liability after birth, increasing vulnerability to obesity, insulin resistance, high blood pressure, and heart disease later in life.