How accurate are fetal weight estimates?

In your final weeks of pregnancy, you may ask your doctor how big he or she thinks your baby will be. Dr David Barrere tells us just how close fetal weight estimates may – or may not – be.

Your question
How accurate are fetal weight estimates late in pregnancy?

The expert answers
Estimation of fetal weight is a complicated and imprecise science. Two methods are typically used:

  1. Physician estimates based on examination — This “guesstimation” is quite often inaccurate. It is not unusual to have larger (macrosomic) babies that are mistaken for average-sized babies. Several factors, including the mother’s size make this method difficult.
  2. Ultrasound estimates using formulas developed by Hadlock or Shepherd can be obtained. The calculations use several fetal anatomy measurements including the biparietal diameter (BPD, head circumference (HC),abdominal circumference (AC), and femur length (FL). The calculations can give an average weight consistent with these measurements, but can be off by as much as 1/2 to 1 pound when performed by an experienced sonographer.

Unfortunately, the science of weight estimations is quite imprecise. Most physicians can detect a smaller than normal or larger than normal baby. This does, however, leave a wide range in what is considered average (6 to 8.5 pounds).


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