Biparietal Diameter and Your Pregnancy Ultrasound

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

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

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

How and When 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 from early in pregnancy through about week 20. People who are considered to be at high risk may need more ultrasounds.

A BPD measurement is useful alongside three other measurements:

  • Head circumference
  • Abdominal circumference
  • Femur bone length (the femur is the thigh bone—the longest bone in the body)

Those three measurements, taken together, help estimate fetal weight and gestational age (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. Your doctor is looking for the BPD measurement, as well as the other measurements, to be within what is considered normal range.

The biparietal diameter measurement increases from roughly 2.4 centimeters at 13 weeks to approximately 9.5 centimeters when a fetus is at term.

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 recommend require further tests. For instance, if your baby's BPD measurements is smaller than usual, that could be a sign of an intrauterine growth restriction or that your baby's head is flatter than usual. If your baby's BPD measurement is larger than expected, it could signal that a health issue, such as gestational diabetes.

A low BPD can be an indication to monitor fetal head growth. Microcephaly can be a concern for women who may have been exposed to the Zika virus. 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 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, movement during the scan, and the skill of the technician. Additional ultrasounds and other tests can help give a better picture of your baby's health.

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