What You Should Know About Biparietal Diameter

Doctor performing ultrasound on pregnant womans belly
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Biparietal diameter (BPD) is one of many measurements that are taken during pregnancy ultrasound procedures. It is a measurement of the diameter across your developing baby's skull, from one parietal bone to the other. The BPD is used to estimate fetal weight and gestational age. The other measurements that are used to calculate fetal weight include abdominal circumference and femur length.

What Are the Parietal Bones?

Every human has two parietal bones—one on the left side of the skull and one on the right side of the skull. Each parietal bone looks like a curved plate that has two surfaces and four sides.

To picture measuring the biparietal diameter, imagine taking a string and placing one end of it at the top of your right ear and the other end of it at the top of your left ear, letting it rest on the top of your head. The length of that string would give you a very rough idea of your biparietal diameter. When your baby is inside your uterus, an ultrasound technician takes this measurement while looking at your developing baby on a computer screen and using digital measuring tools.

How and When Biparietal Diameter (BPD) Is Measured

The BPD measurement is usually taken during standard ultrasounds in pregnancy. Most women have anywhere from one to three ultrasounds (also known as sonograms), usually through about week 20. Women who are considered to be at high risk may need more ultrasounds.

A BPD measurement is useful alongside three other measurements:

  • A head circumference measurement
  • An abdominal circumference measurement
  • The length of the femur bone (the thigh bone—the longest bone in the body).

Those three measurements together help estimate fetal weight and how far along the pregnancy is. The BPD measurement also gives you and your doctor a sense of how your developing baby's brain is growing.

The biparietal diameter measurement tends to increase from roughly 2.4 centimeters at 13 weeks to approximately 9.5 centimeters when a fetus is at term. Your doctor is looking for the BPD measurement, as well as the other measurements, to be within what is considered normal range.

Taking a biparietal diameter measurement late in pregnancy is not considered to be as reliable in predicting gestational age. Between week 12 and week 26 of pregnancy, BPD tends to be accurate for predicting gestational age within 10 to 11 days. However, after week 26 of pregnancy, it may be off by as much as three weeks. Other studies show that BPD becomes less accurate after week 20. 

When BPD Is Outside of the Normal Range

If your baby's results are outside of a normal range, your doctor may require further tests to make sure that you and your baby are healthy. For instance, if your baby's measurements are on the small side, that could be a sign of an intrauterine growth restriction or it could mean that your baby's head is flatter than usual. On the other hand, if your baby's measurements are on the larger side, it could signal that you have a health issue, such as gestational diabetes.

Microcephaly can be a concern for women who may have been exposed to the Zika virus. A low BPD can be an indication to monitor fetal head growth. If the BPD falls two standard deviations below the mean, the head is considered to be excessively flat and microcephaly is suspected. Microcephaly will have other indications, such as the appearance of the head and other measurements. The CDC recommends fetal ultrasounds every three to four weeks for women who have a confirmed or possible Zika virus infection.

A Word From Verywell

It can be concerning if you get ultrasound results and your developing baby has any that are outside of the normal range. But there can be many reasons for this to occur on a single ultrasound, including the position of the fetus and movement during the scan. Your doctor will be looking at all of the results to see if they indicate more testing.

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