When is a baby too small? Managing third-trimester fetal growth restriction
July 6, 2021
The third trimester is generally considered the home stretch of pregnancy. For most patients, it’s a less stressful time regarding fetal health, since the risk of miscarriage drops significantly after the first trimester and most patients have had the opportunity to learn about the health of their baby through second trimester ultrasounds and screening tests.
However, it’s not uncommon for mild to moderate growth disruptions to occur in the third trimester. Often, these healthy babies are simply small because their parents are small in stature or they were small babies themselves. But in approximately one-third of small babies, we find a potentially serious health problem called fetal growth restriction (FGR), which is associated with low birthweight, developmental problems, and stillbirth.
FGR refers to babies who are below the 10th percentile for estimated fetal weight, which means they’re smaller than approximately 90 percent of babies at the same gestational age. There are many potential causes of FGR, which generally fall under maternal, fetal, or placental complications.
Every patient who gets prenatal care is screened for FGR at every visit by taking regular belly measurements. We’re checking your fundal height – from the top of your uterus to your pubic bone. This number helps estimate the baby’s size and whether it is growing as expected.
After 20 weeks, the fundal height should correlate approximately with how long you’ve been pregnant. For example, it should measure approximately 20cm at 20 weeks’ gestation, 30cm at 30 weeks, etc. If your baby measures smaller than expected, we’ll confirm its size with an ultrasound.
“Generally, growth restriction is associated with either maternal risk factors, fetal anomalies, or a problem with the placenta – and sometimes a combination of these situations. Some risk factors can be controlled, and some cannot.”
Is my baby small-but-healthy? Or could it be FGR?
By definition, babies diagnosed with FGR fall below the 10th percentile for estimated fetal weight and fundal height. Even if the baby is growing steadily, they may be diagnosed with FGR if they are below that benchmark.
But following these definitions FGR is sometimes over-diagnosed. Longstanding best practices data show that babies in the 3rd percentile or lower are more likely to have an FGR-related complication, and those above the 3rd percentile are more likely to be constitutionally small (based on non-health-related family history of smaller stature).
The UT Southwestern MFM team follows standard of care protocols for babies below the 10th percentile. We investigate every scenario to avoid taxing patients with undue stress related to overdiagnosis and over-management of FGR. Our goal is to understand:
- Why your baby is small,
- Whether your pregnancy is at risk of serious complications, and
- Whether early delivery or continued monitoring is the safest choice for you and your baby.
To answer these questions, we rely on two diagnostic tools: amniotic fluid measurement and Doppler ultrasound examination of the umbilical cord. These evaluations may help determine the origin of the FGR and guide further management.
What can cause FGR?
Generally, growth restriction is associated with maternal risk factors, fetal anomalies, or a problem with the placenta – and sometimes a combination of these situations. Some risk factors can be controlled, and some cannot.
Maternal risk factors
Some of the most common maternal risk factors for FGR include:
- Certain autoimmune conditions, such as lupus
- Certain viral infections, such as rubella or malaria
- Pregestational diabetes
- Having twins or more
- Hypertension (high blood pressure)
- Smoking, drinking alcohol, and substance use
While it is tough to quit smoking, it’s best to quit before becoming pregnant. However, quitting smoking during pregnancy is proven safe and beneficial for you and the baby. Your prenatal care provider can connect you with effective, judgment-free smoking cessation or substance use treatment designed specifically for pregnant patients.
Heart conditions are among the most common fetal problems that cause FGR. Genetic abnormalities such as trisomy 21 (Down syndrome) also tend to result in babies that are smaller compared with the general population. Fetal anomalies are also more frequently associated with stillbirth.
Problems with the placenta
The placenta, like any organ, can develop improperly. Sometimes the umbilical cord is inserted in the wrong place on the placenta or only one umbilical artery forms develops in the cord instead of two, situations associated with reduced placental blood flow to an otherwise healthy baby.
In rare cases, we see a placental tumor called a chorioangioma, disrupting normal function. Placental issues are often the cause of late-onset growth complications.
How is FGR diagnosed?
The diagnosis is based on ultrasound, and most centers use the 10th percentile cutoff to raise the alarm of possible fetal growth restriction. The next step is to have a discussion with the patient about her health history and her prior obstetrical history.
If a patient’s previous baby was less than 6 pounds at birth but was full-term and healthy, her current baby simply may be constitutionally small. If this is her first, or if previous babies were bigger, we will be more concerned about FGR. In either case, we will check the amniotic fluid and umbilical blood flow to help in the evaluation of placental function.
What happens next?
If the amniotic fluid and Doppler ultrasound are normal, we’ll watch the baby over the next few weeks and, if steady growth continues with no new concerns, that’s a good sign. The actual delivery timing will depend on how well the baby continues to grow and whether the amniotic fluid and the Doppler testing remain normal.
We generally recommend delivering the baby as close to the due date as possible to reduce the risk of needing long-term, specialized care after delivery. If the baby is:
- Between the 3rd and 10th percentiles for size, we try to get them to 38 weeks gestation.
- Smaller than the 3rd percentile, we try to get them to 37 weeks.
Often, our specialized team can safely get babies to 38 or 39 weeks before delivery, provided they have no other risk factors. Centers with less FGR management expertise tend to deliver earlier to avoid the risk of stillbirth. However, the earlier a baby arrives, the longer they typically stay in the neonatal intensive care unit (NICU).
Since the causes and severity of IUG can be so varied, your care plan will be customized based on your needs. Some patients are hospitalized for daily fetal heart rate monitoring. If you are hospitalized, we may recommend a complete course of steroids to help support fetal lung development before delivery.
If you end up needing early delivery, it does not mean an automatic cesarean section (C-section). Many babies are otherwise healthy except for being small, so it is likely safe to try vaginal delivery. Babies that are severely growth restricted or that have abnormal Doppler patterns may not do well with labor, so C-section may be indicated. A discussion with your doctor is always recommended.
What care will the baby need after birth?
Depending on how premature the baby is and the severity of its health problems, the newborn may need to stay in the NICU. Sometimes, these babies have trouble breastfeeding at first because they burn so many calories trying to latch and nurse.
Babies who are growth restricted often need many extra calories – more than breast milk or regular formula can provide. Our NICU specialists will work with nutritionists to supplement your baby’s diet with specialized, calorie-rich formula.
Many babies who start out being bottle-fed after growth restriction can learn to latch and breastfeed once they grow stronger. Patients who want to breastfeed are welcome to begin pumping to start building their milk supply.
It can be scary to learn that your baby is smaller than expected. Sometimes, babies simply grow on their own developmental curve. With specialized care during pregnancy and after delivery, most babies diagnosed with FGR in the third trimester have good outcomes.