Ultrasound can help us evaluate whether a baby is growing as expected during pregnancy. Some babies measure smaller than expected, while others measure larger than expected.
Understanding how fetal growth is estimated can make terms like estimated fetal weight, growth percentile, fetal growth restriction, and macrosomia easier to understand.
We cannot directly weigh a baby before birth, so ultrasound measurements are used to estimate fetal weight. During a growth ultrasound, several measurements are taken, including:
Head circumference (HC) – measurement around the baby's head
Biparietal diameter (BPD) – measurement across the baby's head
Abdominal circumference (AC) – measurement around the baby's abdomen
Femur length (FL) – measurement of the baby's thigh bone
These measurements are combined using a mathematical formula to calculate an estimated fetal weight (EFW).
An ultrasound estimate is not an exact measurement of the baby's weight. There is a margin of error, particularly later in pregnancy and at the extremes of fetal size. Rather than focusing on a single estimated weight, we consider the baby's measurements, growth percentile, growth over time, and other ultrasound and pregnancy findings when evaluating fetal growth.
Estimated fetal weight and individual measurements can be compared with expected measurements for other babies at the same gestational age. This is reported as a percentile.
For example, a baby measuring at the 10th percentile is estimated to be larger than about 10% of babies and smaller than about 90% of babies at the same point in pregnancy.
A percentile by itself does not tell us whether something is wrong. Some babies are naturally smaller or larger than average. We also consider how the baby has been growing over time, individual measurements such as the abdominal circumference, placental blood flow when indicated, amniotic fluid, and other pregnancy factors.
Fetal growth restriction (FGR) means that a baby is measuring smaller than expected and may not be reaching its expected growth potential. You may also hear the older term intrauterine growth restriction (IUGR).
FGR is generally diagnosed when the baby's estimated fetal weight or abdominal circumference measures below the 10th percentile for gestational age. However, not every small baby has a problem. Some babies are simply constitutionally small and healthy, while others are small because the placenta or another condition is limiting their growth.
There are many reasons a baby may measure smaller than expected. Sometimes a baby is simply constitutionally small, meaning they are healthy but naturally smaller because of genetic factors such as having smaller parents.
In other cases, fetal growth may be affected by the placenta, maternal health conditions, or conditions affecting the baby. Factors associated with FGR can include:
Placental insufficiency, when the placenta is not able to support fetal growth as effectively
High blood pressure or preeclampsia
Certain maternal medical conditions, including preexisting diabetes with vascular disease
Low maternal weight gain or inadequate nutrition
Smoking, nicotine, or other substance exposures
Certain fetal genetic or chromosomal conditions
Some infections during pregnancy
Sometimes no specific cause is identified.
If FGR is diagnosed, additional monitoring is usually recommended. This may include:
Growth ultrasounds to follow the baby's growth and measurements over time.
Umbilical artery Doppler studies, performed during an ultrasound, to evaluate blood flow through the umbilical cord and placenta. These measurements help us assess how well the placenta is supporting the baby and can help guide how closely the pregnancy should be monitored and when delivery may be recommended.
Nonstress tests or biophysical profiles to monitor the baby's well-being, particularly later in pregnancy.
The amount and frequency of monitoring depends on how small the baby is, Doppler findings, gestational age, growth over time, and whether other pregnancy complications are present.
When FGR is related to placental insufficiency, there may come a point when the placenta can no longer support the baby's needs as well inside the uterus. The goal is to balance the benefits of allowing more time for growth and maturity against the risks of continuing the pregnancy.
Depending on the baby's size, growth, Doppler studies, fetal testing, and other pregnancy complications, delivery may therefore be recommended before the due date. Babies with FGR may also be less able to tolerate the stress of labor, which can increase the chance that a cesarean delivery becomes necessary for concerns about the fetal heart rate.
Some babies measure larger than expected for gestational age. You may hear several different terms used to describe this:
Large for gestational age (LGA) generally means the baby's estimated weight is above the 90th percentile for gestational age.
Macrosomia refers to a very large baby based on estimated or actual weight rather than percentile. A birth weight of more than 4,000 grams (8 lb 13 oz) or 4,500 grams (9 lb 15 oz) is commonly used to define macrosomia, with risks increasing as birth weight increases.
Ultrasound estimates of fetal weight become less precise as babies get larger, so an ultrasound cannot tell us exactly what a baby will weigh at delivery. Instead, the estimated size is considered along with growth over time, maternal health, and other pregnancy factors.
Larger fetal size may simply reflect normal genetic variation. It is also more common with factors such as maternal diabetes, higher prepregnancy weight, excessive pregnancy weight gain, previous delivery of a large baby, and pregnancies that continue beyond the due date.
Most patients with a baby measuring large can still attempt a vaginal delivery. However, as fetal size increases, there is a greater risk of labor abnormalities or cephalopelvic disproportion (CPD). CPD means that the baby may be too large, or may not be positioned in a way that allows the baby to pass safely through the pelvis. This may become apparent when labor does not progress or the baby does not descend as expected, increasing the likelihood of cesarean delivery.
Larger babies also have an increased risk of shoulder dystocia, an unpredictable delivery complication in which the baby's head delivers but one or both shoulders become difficult to deliver through the pelvis. When this happens, the obstetric team uses specific maneuvers to help release the shoulders and complete the delivery. Most babies do well, but shoulder dystocia can occasionally cause injuries such as a broken collarbone or injury to the nerves that control the arm and shoulder (brachial plexus injury). Rarely, a prolonged delay in delivery can result in more serious complications from decreased oxygen. The risk of shoulder dystocia increases as birth weight increases and is higher when maternal diabetes is present.
Not necessarily. Because ultrasound estimates of fetal weight are imperfect, most patients with suspected LGA or macrosomia can still plan for vaginal delivery.
When the estimated fetal weight is particularly high, however, a planned cesarean delivery may be considered because of the increasing risk of shoulder dystocia and birth injury. ACOG recommends considering planned cesarean delivery when the estimated fetal weight is:
5,000 g (about 11 lb) or greater in a patient without diabetes
4,500 g (about 9 lb 15 oz) or greater in a patient with diabetes
These are not absolute cutoffs requiring cesarean delivery. The estimated fetal weight, accuracy of the estimate, diabetes status, obstetric history, and individual preferences are all considered when making a delivery plan.
If your baby is measuring larger than expected, your obstetrician may discuss the option of delivery around 39 weeks rather than waiting for spontaneous labor, particularly when there is concern that the baby may continue to grow significantly.
The decision is individualized and may depend on the estimated fetal weight and growth pattern, whether diabetes is present, your previous deliveries, your cervical exam, and your preferences. Delivery for suspected macrosomia alone is generally not recommended before 39 weeks.