A baby can stop growing or grow too slowly in the womb when something disrupts the supply of oxygen and nutrients it depends on. This is called fetal growth restriction (FGR), and it affects babies whose estimated weight falls below the 10th percentile for their gestational age. The most common cause is a problem with the placenta, but genetic conditions, infections, and certain maternal health issues can also play a role. Understanding the specific causes can help you make sense of what’s happening and what your care team is looking for.
How the Placenta Can Fall Short
The placenta is the baby’s lifeline. It delivers oxygen, glucose, and amino acids from your bloodstream to your baby’s, and it carries waste back out. When the placenta doesn’t develop properly or its blood supply becomes compromised, the baby simply can’t get enough fuel to grow at a normal rate. This is called placental insufficiency, and it’s the single most common cause of growth restriction in singleton pregnancies.
Several things can go wrong. Early in pregnancy, the placenta anchors itself into the uterine wall by remodeling the spiral arteries that feed it. If that remodeling is incomplete, blood flow to the placenta stays lower than it should be for the rest of the pregnancy. The thickness of the placental barrier also matters: nutrients like glucose cross through specialized transport channels, while oxygen moves by simple diffusion. Both processes depend on adequate blood flow and a healthy placental lining. When the placenta is underperfused, the baby receives less of everything it needs, and growth slows or stalls.
Conditions that damage blood vessels tend to impair placental function. Preeclampsia (pregnancy-related high blood pressure with organ involvement), chronic hypertension, and diabetes can all reduce blood flow to the placenta over time. Smoking and cocaine use have similar effects, constricting the small arteries that supply the placental bed.
Chromosomal and Genetic Causes
Chromosomal abnormalities account for roughly 15 to 20 percent of fetal growth restriction cases. Among those, aneuploidy (having an extra or missing chromosome) explains about 7 percent, with trisomy 18 being the most frequent genetic cause. Trisomy 13 and trisomy 21 (Down syndrome) can also be involved, though growth restriction is more consistently severe with trisomy 18.
Beyond full chromosomal abnormalities, smaller genetic changes, such as microdeletions or duplications too small to appear on a standard karyotype, can also limit a baby’s growth potential. These are sometimes identified through more detailed genetic testing like chromosomal microarray analysis, which your provider may recommend if growth restriction is detected and no other cause is obvious.
Infections That Affect Fetal Growth
A group of infections collectively known as TORCH can cross the placenta and directly harm the developing baby. The acronym stands for toxoplasmosis, other infections (including HIV, syphilis, parvovirus B19, chickenpox, and Zika), rubella, cytomegalovirus (CMV), and herpes simplex virus. Because the baby’s immune system isn’t mature enough to fight these infections, they can interfere with organ development and slow growth considerably.
CMV is one of the more common culprits. Many adults carry it without knowing, but a first-time infection during pregnancy, or reactivation of a previous one, can cause significant growth restriction along with other complications. Toxoplasmosis, typically contracted from undercooked meat or cat feces, is rarer but can have similar effects on fetal development.
What You Might Notice
Growth restriction is tricky because you often can’t feel it happening. Your baby may still move, and you may feel fine physically. The most common way it’s detected is through routine prenatal measurements. At each visit, your provider measures the distance from your pubic bone to the top of your uterus (called fundal height). If that measurement is smaller than expected for your week of pregnancy, it can signal that the baby isn’t growing on track, prompting an ultrasound.
One thing you can pay attention to is fetal movement. While the nature of movements changes as pregnancy progresses (less kicking, more rolling and stretching as space gets tight), the number of movements should not noticeably drop. There’s no universally agreed-upon count that defines “too few,” but your own sense of what’s normal for your baby matters. If movements feel significantly reduced or absent, contact your maternity provider that same day rather than waiting.
How Growth Restriction Is Diagnosed and Monitored
An ultrasound is the primary tool for diagnosing FGR. It estimates the baby’s weight based on head circumference, abdominal circumference, and thigh bone length. If the estimated weight falls below the 10th percentile for gestational age, growth restriction is suspected. Not every small baby has a problem, though. Some are simply constitutionally small, meaning they’re healthy but genetically predisposed to be on the smaller side. These babies are considered “small for gestational age” (SGA) but don’t have true growth restriction.
To distinguish between a healthy small baby and one that’s genuinely compromised, doctors use Doppler ultrasound to measure blood flow through the umbilical artery. This test assesses resistance in the placental blood vessels. In a healthy pregnancy, blood flows continuously through the umbilical cord, even between heartbeats. When the placenta is failing, that flow between heartbeats (called end-diastolic flow) diminishes. In severe cases, it disappears entirely or even reverses direction, which indicates that more than 70 percent of the small arteries in the placenta are blocked. Reversed flow is a sign of advanced placental compromise and is often associated with very severe growth restriction, where the baby’s weight drops below the 3rd percentile.
Doppler monitoring has a meaningful impact on outcomes. In high-risk pregnancies where growth restriction is suspected, using umbilical artery Doppler reduces the likelihood of perinatal death by roughly 29 percent compared to not using it. It also helps providers avoid unnecessary early deliveries, reducing rates of labor induction and cesarean delivery when the baby is actually coping well.
What Happens After Diagnosis
Once growth restriction is confirmed, the core question becomes timing: how long can the baby safely stay in the womb versus when does the risk of staying outweigh the risk of being born early? The answer depends on how severe the restriction is, what the Doppler readings show, and how far along the pregnancy is. Mild cases detected near term may simply be delivered a few weeks early. Severe cases detected much earlier require close surveillance, sometimes with Doppler assessments multiple times per week, to watch for signs that the baby is deteriorating.
There’s no treatment that reverses placental insufficiency once it’s established. The management is about monitoring and making the best decision about when to deliver. In some cases, corticosteroids are given to help mature the baby’s lungs in anticipation of an early birth.
Long-Term Health for Growth-Restricted Babies
Most babies born with growth restriction do well, especially when the condition is detected and managed during pregnancy. But FGR does carry long-term health implications that extend well beyond infancy. Children affected by growth restriction have a higher risk of cardiovascular disease, high blood pressure, impaired insulin sensitivity (which can progress to type 2 diabetes), and kidney dysfunction later in life. Neurodevelopmental effects are also possible, including a higher incidence of attention-deficit/hyperactivity disorder and anxiety.
In adolescence and adulthood, the risks expand to include early puberty, reduced fertility, polycystic ovary syndrome, elevated cholesterol, and accelerated hardening of the arteries. One important finding is that excessive “catch-up growth” in early childhood, where a small baby gains weight very rapidly, may actually worsen the metabolic and cardiovascular risks rather than help. Gradual, steady growth after birth appears to be more protective.
These risks don’t mean every growth-restricted baby will develop these conditions. They do mean that pediatricians and family doctors should be aware of the birth history so they can screen appropriately as the child grows.