Fetal Biometry

Fetal biometry is the measurements your pregnancy care provider uses during an ultrasound to assess how a fetus is growing. Irregular results usually mean more testing is necessary.


What is fetal biometry?

Fetal biometry is a test that measures the size of a fetus during pregnancy using ultrasound. It consists of several measurements that tell your pregnancy care provider how the fetus is growing for its gestational age and if there are signs of growth problems. It helps diagnose fetal growth disorders. These disorders can lead to pregnancy complications later on.

Why is fetal biometry important?

Fetal biometry is part of a prenatal ultrasound. It helps determine if the fetus’s size matches up to its estimated gestational age. The estimated gestational age is an estimate of how far along a pregnancy is in weeks.

If the fetus seems smaller or larger than its gestational age, it tells your provider that more tests may be necessary to see why the fetus is measuring small or large. This could be due to an underlying health condition in the birth parent, issues with the placenta or a congenital abnormality in the fetus.

Keep in mind that just because results of fetal biometry show whether a fetus measures ahead or behind for its age, it doesn’t mean something is wrong. It can be quite normal to have results that are irregular.


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Test Details

What measurements make up fetal biometry?

Fetal biometry is a test to estimate the size of a fetus during pregnancy. By measuring certain body parts, your provider is able to paint an overall picture of the fetus’s size and match it up to what’s considered typical for its age.

The parameters this test measures include the following:

  • BPD (biparietal diameter): The diameter of the head.
  • HC (head circumference): The length going around the head.
  • AC (abdominal circumference): The length going around the belly.
  • FL (femur length): The length of the femur bone.
  • CRL (crown-rump length): The length from the top of the head to the bottom. This measurement is more accurate in the first trimester. After 12 weeks of pregnancy, the parameters above are better at giving an overall assessment of growth.

How is fetal biometry performed?

Fetal biometry is a medical term for the measurements your provider is looking at in a routine prenatal ultrasound. Along with checking things like the amount of amniotic fluid and blood flow through the umbilical cord, an ultrasound technician uses their tools to take measurements that indicate if the fetus is the size it should be for its age. During a prenatal ultrasound, a technician moves a transducer across your belly. If you’re less than 12 weeks into pregnancy, they may use a transvaginal ultrasound.

If you’re watching on an ultrasound monitor, you may notice your technician clicking and dragging lines across the screen. This is how they take measurements of the fetus and other parts of your uterus.


How accurate is fetal biometry?

Fetal biometry is very accurate in early pregnancy. It’s one of the most reliable ways a healthcare provider determines how a fetus is growing. Your provider will typically take these measurements during routine ultrasounds in the first and second trimester. If you have a pregnancy complication or are expecting multiples, your provider will take these measurements more often throughout your pregnancy. Fetal biometry tends to be less accurate after your second trimester.

Results and Follow-Up

What are normal ranges for a fetal biometry?

Typical ranges for fetal biometry vary and many factors go into determining what’s “normal.” Each parameter has an average measurement at different gestational ages. Further, each pregnancy is different and your provider will take other factors into account when they assess your results.

In centimeters (cm), the median (average) ranges for fetal biometry measurements at around 20 weeks gestation are:

  • Head circumference (HC): 17.5cm.
  • Abdominal circumference (AC): 14.9cm.
  • Biparietal diameter (BPD): 4.9cm.
  • Femur length (FL): 3.2cm.

Since the likelihood of getting a result that’s exactly the median is rare, you’ll also get percentile values. A percentile is a way to compare people with similar characteristics. In this case, healthcare providers use percentiles to show the general size range of fetuses of the same gestational age.

For example, your results may show the fetus is in the 75th percentile for its abdominal circumference. While it’s higher than average, it’s not necessarily a cause for worry. Most healthcare providers aren’t concerned unless a fetus measures less than the 10th percentile or higher than the 90th percentile.

Since test results are so unique, it’s always best to talk to your provider about your test results and what they mean for you. And, since many factors play a role, it’s very hard to make general statements about what’s normal.

What does an abnormal result mean?

If the results of your ultrasound are irregular, your pregnancy care provider will recommend additional ultrasounds. This helps them monitor the fetus’s growth over time to see if anything changes. Looking at trends in your results or how your results change over time is the best way to confirm fetal biometry. Additionally, they may look at the results of your blood work or urinalysis (a test that uses a sample of your pee) to see if these provide any insights as to why the fetus’s measurements were irregular.

A fetus measuring smaller than average may indicate FGR (fetal growth restriction) or problems with the placenta. A fetus that measures larger than average may point to the birth parent having gestational diabetes.

Try not to worry. It’s common for a fetus to measure small or large. In most cases, the fetus is born healthy and at a size and weight in a normal range. Irregular test results on a single ultrasound aren’t a reason to panic. Your pregnancy care provider will let you know about additional monitoring and what comes next.

A note from Cleveland Clinic

Fetal biometry is measurements your provider takes during an ultrasound to assess the growth of a fetus. You’ll likely never hear the term fetal biometry during your pregnancy, but you may see it on your ultrasound report. It’s a routine measurement that shouldn’t cause worry, but the results could mean the fetus is measuring smaller or larger than normal for its age. Discuss any questions you have about ultrasounds or other tests with your pregnancy care provider. They’re there to help you and answer your questions.

Medically Reviewed

Last reviewed on 06/20/2023.

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