Can They Tell How Big the Baby Is by Ultrasound?

Woman getting an ultrasound
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Ultrasound exams are notoriously inaccurate for predicting the weight of your baby. The ultrasound gives 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 the weight via ultrasound, and the results can vary widely. It is generally not advisable to make decisions about induction 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 ultrasound measurements like your baby's head circumference (via biparietal diameter), abdominal circumference, femur length, and others.

Various programs add the sex of the baby, the gestational age, and other factors into the mix. For example, babies of diabetic mothers 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. This is not an accurate tool for predicting the true weight of the baby, though some practitioners are better than others at making weight estimates this way.

Estimate-Based Inductions or Cesareans

When the estimated weight suggests the child is large for gestational age, some doctors will plan an induction or scheduled cesareans. In general, a baby who is estimated to be 10 pounds or more—known as macrosomia—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 may 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.

While cesareans are sometimes performed for large babies, the babies may actually be several pounds under the pre-birth estimate. This is why many people do not recommend using this estimate to make decisions about the mode of birth. Sometimes estimates are under as well.

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. Does your practitioner have a specific concern that only an ultrasound can answer? Is there something going on? Or is it a routine procedure done on most women in the practice.

Some things that your practitioner may want to look at in the third trimester include, but are not limited to:

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.

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