Fetal Biometry (Twins)

Who Gets the Test?

Most pregnant women get a routine fetal biometry.

What the Test Does

Fetal biometry measures your twins' sizes. During an ultrasound, your doctor measures the babies' heads, bodies, and thigh bones. It helps show your babies' development.

How the Test Is Done

Fetal biometry is a measurement taken during a standard ultrasound. During the ultrasound, a technician puts a gel on your belly, and then gently moves the ultrasound wand on your stomach to see images of your twins.

What to Know About Test Results

Your doctor will use the fetal biometry to estimate your babies' age, sizes, weights, and growth. You may get a report after your scan with the measurements. The report may include:

  • BPD (biparietal diameter), the diameter of your babies' heads
  • HC (head circumference), the length going around your babies' heads
  • CRL (crown-rump length), the length from the top of the heads to your babies' bottoms, measurement taken in the first trimester
  • AC (abdominal circumference), the length going around your babies' bellies
  • FL (femur length), the length of a bone in your babies' legs

If your babies' results are unusual, your doctor will suggest further testing. Small size may be a sign of intrauterine growth restriction (IUGR.) Large size may be a sign that the mother has a health problem, such as gestational diabetes.

How Often the Test Is Done During Your Pregnancy

Your doctor will estimate the size of your babies during standard ultrasounds. Most women get one to three ultrasounds when they're pregnant with singletons. With twins, it is common to have ultrasounds more often.

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WebMD Medical Reference Reviewed by Traci C. Johnson, MD on June 25, 2018



March, M.I. Clinical Obstetrics & Gynecology, March 2012.

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