Ultrasound and Weight of Your Baby

Reviewing the Accuracy of Ultrasound to Measure the Infant's Weight

Apr 3, 2008

Ultrasound is used for several purposes during pregnancy including estimating the baby's weight. Controversy remains about the accuracy of ultrasound.

If you are like most expectant mothers today, you will have several ultrasounds during your pregnancy. Ultrasound is often used to confirm pregnancy in the early weeks. It is also used to estimate your due date within a two week window. Ultrasound during pregnancy can be used as a screening tool for the health and development of your baby. Late in pregnancy, ultrasound is often used as a tool to estimate the weight of the baby. It is this last estimation that is controversial as the weight estimate is based on averages and may not be 100% accurate.

There are four measurements that are commonly taken with an ultrasound in order to estimate the weight of the baby. These are the biparietal (sideways across the baby's head) diameter, the baby's head circumference, the abdominal circumference and the length of the baby's femur (thigh bone.) Using these measurements, the sonographer uses various equations to come up with a weight estimate of your baby.

Remember however that this is only a weight estimate. The measurements cannot guarantee that your baby will actually be the same weight as the one that is estimated. In some cases, the weight estimate can be as much as 1 pound off in terms of accuracy. The weight estimate could be thrown off simply because your baby has longer legs than the average baby for her size.

In fact many times decisions are made based on these weight estimates that affect both mothers and babies. For example, if the ultrasound weight estimate shows that your baby is approximately 9 pounds, your health care provider might be inclined to recommend a labor induction or even a cesarean if there is concern the baby is too large to be born vaginally.

The reality is that all medical testing, including ultrasound used to determine the weight of your baby, is not 100% accurate. Not only is it recommended that you ask your provider about the accuracy of ultrasound testing to estimate weight before you make any decisions, but remember that the size of your baby may not be prohibitive in having a vaginal birth. It is important to be informed about ways to encourage labor to happen spontaneously and how to work with your body even if there is suspicion that your baby is big.

Have you been told your baby is big according to the ultrasound estimate? What decisions are you planning to make? Share your story with us.

The copyright of the article Ultrasound and Weight of Your Baby in Prenatal Health is owned by Brenda Lane. Permission to republish Ultrasound and Weight of Your Baby in print or online must be granted by the author in writing.