Understanding Fetal Size: What a Large Belly Means at 34 Weeks

Question
Ultrasound @ 34 weeks suggests baby has large tummy so big for dates.  My daughter has to have another GTT although first one several weeks ago was negative.  Why would a big tummy indicate the baby's size if head and legs are normal size?  Thanks.

Answer
Dear Barbara,

Biometry is used to determine fetal age. By late in the 3rd trimester, measurements are accurate to only within a 2-3 week window. In general, once the crown-rump measurement (CRL) is no longer reasonable to obtain, the averaging of several anatomic measurements is more accurate than the use of one measurement alone. The four most common measurements taken during biometry are the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). (The big tummy would be the AC) Results outside of the expected outcome should be further evaluated, to check for fetal anomalies.

Normal fetal size and weight values are expected between the 10th and 90th percentiles. Fetal size or weight below the 10th percentile is considered small for gestational age (SGA). A fetus with measurements above the 90th percentile is considered large for gestational age (LGA), and may be an indication of fetal response to maternal gestational diabetes. The diagnosis of an SGA or LGA fetus impacts the future obstetric management of the pregnancy, and assists in avoiding complications related to these conditions. It sounds like the OB doctor is following protocol and simply making sure that any chance of complication is minimized.

I hope this has helped you and answered your question. I wish you well.

Brenda