IUGR, or intrauterine growth restriction, means a baby is growing more slowly than expected during pregnancy. It’s diagnosed when a baby’s estimated weight falls below the 10th percentile for their gestational age, meaning the baby is smaller than 90% of other babies at the same stage of pregnancy. The term is now often used interchangeably with “fetal growth restriction” (FGR), and if your doctor mentioned either one, they’re talking about the same concern.
How IUGR Is Different From a Small Baby
Not every small baby has growth restriction. Some babies are simply small because their parents are small, and they’re perfectly healthy. The medical term for any baby born below the 10th percentile is “small for gestational age” (SGA), but that label alone doesn’t tell you whether something is wrong. IUGR specifically implies that the baby isn’t reaching its growth potential because something is interfering with normal development.
The distinction matters because a constitutionally small baby doesn’t need the same level of monitoring as one whose growth has slowed due to a problem with nutrient or oxygen delivery. When doctors suspect IUGR, they use blood flow measurements and other tools to figure out which category a small baby falls into. Babies whose estimated weight drops below the 3rd percentile face the highest risk of serious complications, which is why many experts now use that stricter cutoff to identify the most concerning cases.
Symmetrical vs. Asymmetrical Growth Restriction
IUGR generally falls into two patterns. In symmetrical growth restriction, the baby’s entire body, including the head, abdomen, and limbs, is proportionally small. This type tends to start early in pregnancy and is more often linked to genetic conditions, chromosomal abnormalities, or infections that affect the baby directly during the first trimester when cells are rapidly multiplying.
Asymmetrical growth restriction is more common and typically develops later, in the second or third trimester. In this pattern, the baby’s head continues to grow at a relatively normal pace while the abdomen lags behind. This happens because the baby’s body redirects blood flow to protect the brain at the expense of other organs, particularly the liver where energy is stored. Asymmetrical IUGR is most often caused by problems with the placenta that limit how much oxygen and nutrition reach the baby.
What Causes IUGR
The causes generally fall into three categories: problems with the mother’s health, problems with the placenta, and problems originating in the baby itself.
On the maternal side, conditions that affect blood flow are the biggest culprits. Preeclampsia (dangerously high blood pressure during pregnancy), chronic hypertension, diabetes, kidney disease, and autoimmune disorders can all reduce the supply of nutrients reaching the baby. Smoking, alcohol use, and drug use during pregnancy also significantly increase the risk. Poor maternal nutrition and very low pre-pregnancy weight play a role as well.
Placental causes include any condition where the placenta doesn’t develop properly or begins to deteriorate. The placenta is the baby’s lifeline, and if it can’t transfer enough oxygen and nutrients, growth slows. Placental abruption (where the placenta partially separates from the uterine wall) and abnormal cord insertion are examples.
Fetal causes include chromosomal abnormalities, congenital infections (such as cytomegalovirus, toxoplasmosis, or rubella), and carrying multiples. Twin and triplet pregnancies naturally demand more resources, and growth restriction in one or both babies is common.
How IUGR Is Detected and Monitored
Most cases are first suspected when a routine ultrasound shows the baby measuring smaller than expected. Once suspected, doctors use Doppler ultrasound to measure blood flow through the umbilical artery, the vessel that carries blood from the baby to the placenta. This is a painless, noninvasive test that tells doctors how much resistance the placenta is putting up against blood flow. Higher resistance means the placenta isn’t working well, and the baby may not be getting enough oxygen.
The most commonly used measurement is called the pulsatility index, which compares the speed of blood flow during heartbeats to the flow between heartbeats. Doctors also look at the cerebroplacental ratio, which compares blood flow in the baby’s brain to flow in the umbilical cord. When the brain is receiving a disproportionate share of blood flow, it confirms the baby is compensating for reduced placental function.
How often you’ll be monitored depends on severity. For mild to moderate IUGR, the typical schedule is a weekly modified biophysical profile (a combination of a non-stress test and amniotic fluid check) along with Doppler studies, plus a growth scan every two weeks. In severe cases, monitoring increases to twice weekly. Growth scans aren’t done more frequently than every two weeks because shorter intervals actually decrease accuracy.
Risks for the Baby at Birth
Babies born with IUGR face several immediate challenges. The most common is hypoglycemia, or low blood sugar. In one study, about 31% of IUGR newborns experienced hypoglycemia, and these babies were nearly five times more likely to have dangerously low blood sugar than normally grown babies. This happens because growth-restricted babies have fewer energy reserves stored in their livers.
Other short-term risks include low Apgar scores (a quick health assessment done in the first minutes after birth), difficulty maintaining body temperature, jaundice, and a higher chance of needing breathing support. IUGR babies also tend to stay in the hospital longer. Research shows their median hospital stay is about nine days compared to five days for babies who grew normally.
Intraventricular hemorrhage, or bleeding in the brain, is another complication that occurs at significantly higher rates in IUGR newborns, particularly in those born prematurely.
Long-Term Health Effects
The consequences of IUGR don’t end at birth. Decades of research, including landmark studies of people born during the Dutch famine of 1944-45, have shown that growth restriction in the womb can reprogram metabolism in ways that increase the risk of chronic disease later in life. This concept, sometimes called the “Barker hypothesis” after the researcher who first described it, suggests that when a fetus is deprived of adequate nutrition, its body adapts by becoming extremely efficient at storing calories and conserving energy.
Those adaptations are helpful during scarcity but become harmful when the person grows up with access to plentiful food. The result is a significantly higher risk of obesity, type 2 diabetes, high blood pressure, and heart disease in adulthood. Estimates suggest that 25 to 63% of adult diabetes, hypertension, and coronary heart disease can be attributed to the combination of low birth weight followed by rapid weight gain during childhood and adolescence.
There are also neurological effects. Animal research has shown that IUGR can cause permanent changes in the brain’s appetite-regulating centers, leading to resistance to the hormones that normally signal fullness. This may partly explain why individuals who were growth-restricted in utero are more prone to overeating and obesity as adults. The association with high blood pressure is particularly strong in males.
Prevention With Low-Dose Aspirin
For women at high risk of preeclampsia, which is one of the leading causes of IUGR, low-dose aspirin can reduce the chance of growth restriction. The U.S. Preventive Services Task Force recommends 81 mg of aspirin daily, started after 12 weeks of pregnancy and ideally before 16 weeks, continuing until delivery. In clinical trials involving over 14,000 at-risk pregnancies, this regimen reduced the risk of growth restriction by about 18%.
Risk factors that qualify someone for aspirin prophylaxis include a history of preeclampsia, chronic hypertension, kidney disease, autoimmune conditions, and carrying multiples. Your provider will assess whether you meet the criteria based on your specific medical history.