What You Should Know About Biparietal Diameter

What the BPD Measurement on Your Pregnancy Ultrasound Means

Doctor performing ultrasound on pregnant womans belly
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Biparietal diameter (also known as BPD) is one of many measurements that's taken during pregnancy.

What Are the Parietal Bones?

To back up for a moment, 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. 

So, the BPD measurement is the diameter across your developing baby's skull, from one parietal bone to the other.

To picture it, 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 "high risk" may need more ultrasounds.

Taking a biparietal diameter measurement late in pregnancy is not considered to be as reliable. For example, some research shows that between week 12 and week 26 of pregnancy, BPD tends to be accurate 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. 

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. Essentially, your doctor is looking for the BPD measurement, as well as the other measurements listed earlier, to be within what is considered normal ranges.

    When BPD Is Outside of 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.


    Baby Med: Fetal and Obstetric Ultrasound Measurements in Pregnancy.

    MacGregor, S, Sabbagha R. Glob. libr. women's med(ISSN: 1756-2228) 2008. Assessment of Gestational Age by Ultrasound.

    Military Obstetrics and Gynecology, Brookside Associates. (2014). 2nd and 3rd Trimester Ultrasound Scanning

    Stanislovsky A. Radiopaedia.org: Biparietal diameter.

    Woo J, Biparietal Diameter. Ob-ultrasound.net: Obstetric Ultrasound--A Comprehensive Guide