Can they tell how big the baby is by ultrasound?

Woman getting an ultrasound
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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.

There are currently more than thirty algorithms used to predict the weight of your baby via ultrasound. Most use common ultrasound measurements like your baby head circumference, abdominal circumference, femur length, and others. Various programs add the sex of the baby, the gestational age, and other factor into the mix. Some are more accurate than others. 

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 also not accurate for predicting the true weight of the baby, though some practitioners are better than others at estimating the weight of the baby.

Many women will have lots of stories about being induced for having suspected large babies, there are even stories about scheduled cesareans simply for the estimated size of the baby.

One mom even related that her practice recommends a scheduled cesarean for any baby that is thought to be over eight pounds. While there are cesareans are done for large babies, sometimes 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.

Sometimes the estimates are under as well.

Amanda explains what happens in her labor, "My doctor wanted to induce because they were concerned that my baby was due that day and I wasn't in labor. I had an ultrasound just before labor. They told me that my baby would be about seven and a half pounds. After he was born, he weighed in at 10 lbs 13 ozs. That was way off!"

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.

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 once piece of the puzzle.

Sources:

Bajracharya J, Shrestha NS, Karki C. Accuracy of prediction of birth weight by fetal ultrasound. Kathmandu Univ Med J (KUMJ). 2012 Apr-Jun;10(38):74-6.

Barel O, Vaknin Z, Tovbin J, Herman A, Maymon R Assessment of the accuracy of multiple sonographic fetal weight estimation formulas: a 10-year experience from a single center.​ J Ultrasound Med. 2013;32(5):815.

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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