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Can they tell how big the baby is by ultrasound?

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Updated May 15, 2014

36 Week 3D Ultrasound Face
Photo © A. Swanepoel
Question: Can they tell how big the baby is by ultrasound?
Answer:

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, making it not advised for most to make decisions about induction and planned cesarean section based solely on estimated fetal weight.

Your midwife or doctor may also try to predict the fetal weight by using their hands during a physical exam of your abdomen. This is also not accurate, though some practitioners are better than others.

Many women will have lots of stories about being induced for having suspected large babies, there are even stories about scheduled cesareans. Where the cesareans are done for large babies, but at birth the babies are several pounds under the prebirth estimate. This is why many people do not recommend using this estimate to make decisions about the mode of birth.

One thing that is show in some studies is that an estimation of a big baby or a fat baby via ultrasound can increase the likelihood that you will have a cesarean. This is thought to be because your practitioner has the "big baby" seed planted in his or her mind already and is less tolerant of variances in labor.

Sources:

Blackwell SC, Refuerzo J, Chadha R, et al. Overestimation of fetal weight by ultrasound: does it influence the likelihood of cesarean delivery for labor arrest? Am J Obstet Gynecol 2009;200:340.e1-340.e3.

Colman A, Maharaj D, Hutton J, Tuohy J. Reliability of ultrasound estimation of fetal weight in term singleton pregnancies. N Z Med J. 2006 Sep 8;119(1241):U2146.

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