Intrauterine Growth Restriction (IUGR)

What is intrauterine growth restriction?

Doctors estimate the baby's weight from ultrasound measurements of the head, belly and thighbone. IUGR, more often called fetal growth restriction (FGR) today, is generally diagnosed when the estimated fetal weight or belly circumference is below the 10th percentile for gestational age.

Not every small baby has a problem. Some are healthy and simply small, like their parents. The challenge is telling a constitutionally small baby from one whose growth is genuinely restricted and who is at risk of low oxygen. Doppler scans, the growth trend over time and amniotic fluid levels help make that distinction.

Types: symmetric vs asymmetric IUGR

In asymmetric IUGR the baby diverts blood to the brain (a "brain-sparing" effect), so the liver and belly grow less.

Doctors also divide FGR into early-onset (before 32 weeks), which is usually more severe and linked to preeclampsia, and late-onset (32 weeks or later), which is more common and milder but can still lead to stillbirth if missed.

Causes and risk factors

Placental causes (most common)

• Preeclampsia and gestational hypertension
• Abnormal placental development or a small placenta
• Partial placental abruption or placental infarcts
• Abnormal cord insertion

Maternal factors

• Smoking, alcohol and drug use, including cocaine
• Chronic high blood pressure, kidney disease, diabetes with blood vessel disease, lupus and antiphospholipid syndrome
• Being underweight or poor nutrition
• Mother over 40, or first pregnancy
• Living at high altitude
• Some medicines, such as certain antiseizure drugs

Fetal factors

• Chromosome conditions such as trisomy 18 or 13
• Genetic syndromes and birth defects
• Congenital infections: cytomegalovirus, toxoplasmosis, rubella, syphilis and Zika
• Twins or multiple pregnancy

Previous history

A previous pregnancy with growth restriction or stillbirth raises the risk in later pregnancies.

How IUGR is found

• Fundal height: from about 24 weeks, the midwife or doctor measures from the pubic bone to the top of the uterus. A measurement 3 cm or more below the number of weeks, or a falling trend, prompts a scan.
• Ultrasound biometry: measures the head, belly and femur to estimate weight. At least two scans 2 to 3 weeks apart show the growth trend.
• Umbilical artery Doppler: checks blood flow in the cord. Reduced, absent or reversed end-diastolic flow signals a struggling placenta.
• Middle cerebral artery and ductus venosus Doppler: show brain-sparing and how the baby's heart is coping, especially in early-onset FGR.
• Amniotic fluid volume: low fluid (oligohydramnios) suggests the baby is producing less urine because of reduced blood flow.
• Further tests in early or severe cases: detailed anatomy scan, genetic testing by amniocentesis, and blood tests for infections.

Management and monitoring

There is no treatment that reliably makes a growth-restricted baby grow. Care aims to prevent stillbirth and avoid unnecessary early delivery.

• Serial ultrasounds every 2 to 4 weeks for growth, and Doppler and fluid checks as often as twice weekly in more severe cases
• Nonstress tests (cardiotocography): monitoring the baby's heart rate pattern
• Biophysical profile: an ultrasound score of movement, tone, breathing and fluid
• Counting fetal movements: report any decreased fetal movement promptly
• Treating the cause: blood pressure control, stopping smoking, managing other conditions
• Steroid injections for the mother if delivery is likely before 34 weeks, to mature the baby's lungs
• Magnesium sulfate before very early delivery (under 32 weeks) to help protect the baby's brain

Timing of delivery

Timing balances the risks of staying in the womb against the risks of prematurity. ACOG and SMFM guidance generally suggests:

Vaginal birth is often possible, with continuous monitoring. A cesarean is chosen when the baby is unlikely to tolerate labor.

Complications

For the baby

• Stillbirth, the main reason for careful monitoring
• Premature birth and its complications
• Low oxygen during labor
• After birth: low blood sugar, low body temperature, high red blood cell count, feeding problems and jaundice
• Longer-term: a higher risk of developmental delays, and of high blood pressure, heart disease and type 2 diabetes in adulthood

For the mother

Complications are usually from the underlying cause, such as preeclampsia.

Outlook

Most babies with mild, late-onset growth restriction do well and catch up in size within the first 2 years. Babies with severe, early-onset FGR face more risks, mainly from prematurity, and benefit from delivery in a hospital with a neonatal intensive care unit.

Prevention

• Stop smoking, alcohol and drug use in pregnancy
• Attend all prenatal appointments, which include fundal height checks
• Low-dose aspirin from 12 to 16 weeks, if your doctor recommends it, for people at high risk of preeclampsia
• Good control of blood pressure, diabetes and other conditions before and during pregnancy
• Plan earlier and more frequent growth scans after a previous growth-restricted baby

Frequently asked questions

What causes intrauterine growth restriction?

The most common cause is a placenta that does not supply enough oxygen and nutrients, often linked to preeclampsia or high blood pressure. Other causes include smoking, drug and alcohol use, maternal kidney disease or lupus, infections such as cytomegalovirus, chromosome conditions and twin pregnancies. Sometimes no cause is found.

Can a baby with IUGR catch up in growth before birth?

Usually not significantly. There is no treatment that reliably increases growth in the womb, although treating high blood pressure and stopping smoking may help. Many babies with mild growth restriction catch up after birth, typically within the first 1 to 2 years.

Will I need to deliver early if my baby has IUGR?

Often, but not always. With normal blood flow and mild restriction, delivery is usually planned around 38 to 39 weeks. Abnormal Doppler results or monitoring push delivery earlier, sometimes before 34 weeks. Your care team weighs the risks of prematurity against staying in the womb.

What is the difference between IUGR and a small baby?

A small for gestational age baby is under the 10th percentile, but many are healthy and simply small by nature. IUGR means the baby is not reaching its growth potential, usually because of a problem such as poor placental function. Doppler scans and the growth trend help tell them apart.

Can IUGR be detected by ultrasound?

Yes. Ultrasound measurements estimate the baby's weight, and repeated scans show the growth trend. Doppler ultrasound checks blood flow in the umbilical cord and brain to show how well the placenta is working, and the amniotic fluid level gives further clues.