Understanding Biparietal Diameter and Your Pregnancy Ultrasound

Expectant parents undergo many procedures to monitor the health and wellbeing of their unborn child, one of which is an ultrasound examination. During an ultrasound, measurements are taken to ensure baby is growing and developing appropriately. The fetal heart rate is charted and measured. Organs are looked at, for size and functionality, and biparietal diameter (BPD) is one of many measurements that is 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 one of several measurements 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.

What Does BPD Mean On an Ultrasound?

BPD, or biparietal diameter, is a measurement that is taken through ultrasound imaging during pregnancy. This occurs during routine examinations and is a measurement of the space between the two sides of the skull. The purpose of this measurement is to help doctors and medical professionals better estimate your unborn baby's weight and gestinational age. It also helps them assess their development.

What Is the Importance of BPD Ultrasound Meaning?

It is important to measure BPD for several reasons, including:

• Monitoring growth: In order to determine if your unborn baby is growing and developing on schedule, their BPD—as well other fetal measurements—are taken. A steady increase in BPD is indicative of "normal" development, while deviations may signal potential issues.
• Estimating age: With BPD, the gestational age of your baby may be estimated.
• Fetal biometry: Fetal biomentry is a set of measurements used to estimate the gestational age, growth, and well-being of your unborn baby. BPD is but one measurement used in this panel. Other paremeters include abdominal circumference, and femur bone length.

How Is BPD Measured? 

The BPD measurement is usually taken during standard ultrasounds in pregnancy. Most people 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. 

What to Do When BPD Is Outside the Normal Range 

If your baby's results are outside of a normal range, your doctor may recommend 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 has other indications, such as the appearance of the head and other measurements.

The Centers for Disease Control and Prevention (CDC) recommends fetal ultrasounds every three to four weeks for women who have a confirmed or possible Zika virus infection.