Can an Ultrasound Show How Big a Baby Is?

Ultrasound exams are notoriously inaccurate for predicting the weight of your baby. An ultrasound can give an estimate of the weight of your baby, but this estimate can be off a pound or more in either direction.

There are multiple ways to predict a baby's weight via ultrasound, and the results can vary widely. It is generally not advisable to make decisions about induction of labor or a planned cesarean section based solely on estimated fetal weight.

How Is Weight Predicted?

There are more than 30 different algorithms used to predict the weight of your baby via ultrasound. Most use common measurements like your baby's head circumference (via biparietal diameter), abdominal circumference, femur length, and others.

Various formulas add the sex of the baby, the gestational age, and other factors into the mix. For example, babies of people with gestational diabetes can be larger due to maternal high blood sugar, and some estimates assume these babies will be larger.

Your midwife or doctor may also try to predict the fetal weight by using their hands during a physical exam of your abdomen using Leopold's Maneuvers, which also helps determine the position of the baby in the uterus.

Estimate-Based Inductions or Cesareans

When the estimated weight suggests that a baby is large for its gestational age, some doctors plan an induction or scheduled cesarean. In general, if a baby is estimated to weigh 10 pounds or more—known as macrosomia—this may warrant a C-section.

The primary concern with vaginal delivery of a large baby is shoulder dystocia. This occurs when the baby's body gets stuck in the birth canal and can cause damage to arm and shoulder nerves. This can result in permanent paralysis of the child's arm muscles as well as injury to the mother.

The challenge is that babies may actually be a pound or more under the pre-birth estimate. Or, they may actually weigh more than the estimate. This is why many people do not recommend using estimated weight to make decisions about the mode of birth.

Third-Trimester Ultrasounds

If you are being asked to do an ultrasound in the last trimester of pregnancy, ask what the ultrasound is being used to tell you. It's important that you understand what your practitioner is looking for.

Do they have a specific concern that only an ultrasound can answer? Is there something going on? Or is it a routine procedure done on most patients in the practice? Some things that your practitioner may want to look at in the third trimester include:

• Amount of amniotic fluid
• Biophysical profile
• Placental location
• Position of the baby

Be sure to talk to your doctor or midwife about their thoughts. Even if your baby is on the larger size, this does not mean that you will be unable to give birth vaginally. The size of the baby is only one piece of the puzzle.