Once parents-to-be find out that their baby has no diagnosed abnormalities on ultrasound, their next questions address two major parental concerns: what's the sex and how much does he weigh. While fetal ultrasound is no substitute for plopping a naked baby onto a scale after birth, by using certain fetal measurements and different formulas, doctors can estimate fetal weight. However, it remains just that -- an estimate, one which can be off by a considerable amount, especially in babies most at risk-- the underweight and the overweight.
Most fetal ultrasound weight estimates take several measurements into account. One is often your baby's abdominal circumference; the other might be his head circumference or his femur length, depending on which formula is used. Three-D ultrasounds have more diagnostic accuracy than 2-D ultrasounds, but they're used less frequently due to the amount of time they take, according to an Austrian study published in the December 2009 issue of "Ultrashall in der Medizin." Depending on the measurements used, the number of babies whose weight was within 10 percent of the ultrasound estimate ranged from 31 to 70 percent in a number of studies, obstetrician Dr. Gerard G Nahum, et al reported in a 2011 Medscape article.
The timing of an ultrasound can also affect its accuracy in predicting your baby's weight at birth. In an Israeli study reported in the February 2007 issue of the "European Journal of Obstetrics, Gynecology and Reproductive Biology," predicted birth weight based on ultrasound studies done early in the third trimester of pregnancy -- between 28 and 34 weeks -- correlated poorly with actual birth weights. The study also found that early third-trimester ultrasounds didn't accurately predict whether a baby would be macrosomic, or larger than normal, or small-for-gestational age. A Turkish study published in the 2013 "Journal of Clinical and Experimental Investigations" of serial ultrasounds done in the third trimester found that late third trimester ultrasounds were more accurate in predicting fetal weight than earlier ultrasounds.
A number of variables besides timing can affect the accuracy of a fetal ultrasound. The patient's body weight, the position of the placenta, the amount of amniotic fluid present, and the position of the baby can all influence the accuracy of the measurements taken to determine fetal weight. The skill of the technician can also affect the results.
One risk of overestimating your baby's weight might be your doctor's increased interest in either inducing labor early or doing a Cesarean section. In a University of Texas Health Science Center study reported in the March 2009 issue of the "American Journal of Obstetrics and Gynecology," mothers with babies whose birth weights were overestimated by 15 percent or more during bedside ultrasounds before labor induction had a higher risk of Cesarean delivery for arrest of labor. Nearly 35 percent of women with fetal weight overestimation had Cesarean delivery for arrest, compared to 13 of those whose birth weights weren't overestimated, even though their durations of labor were similar.