Scans & Tests
Updated on 11 May 2023
Biparietal diameter (BPD) is a fundamental biometric parameter used in prenatal ultrasound to evaluate fetal growth and development. BPD is a critical measurement for estimating gestational age, and it can also help identify fetal abnormalities. In this article, we'll delve into the significance of BPD in pregnancy and explore what it can tell us about the developing fetus.
The biparietal diameter (BPD) is one of the many measurements taken during pregnancy ultrasounds. BPD in ultrasound measures how broad a baby's head is from one parietal bone to the other as it grows. Additionally, BPD in pregnancy, with the head circumference (HC), the abdominal circumference (AC), and the length of the femur (FL), is also used to estimate the weight of the fetus.
The human skull consists of two parietal bones, one located on the left and the other on the right side. These bones exhibit a curved plate-like structure with four sides and two flat surfaces. To estimate the biparietal diameter of the skull, one can imagine placing a string from the left ear to the right ear, and letting it rest on the top of the head.
BPD in pregnancy is most accurate between the 14 and 20 weeks of pregnancy. After this point, there is more change. This measurement is typically taken by ultrasound technicians, who use digital measuring tools to examine the growth of a fetus displayed on a computer screen.
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Now that you know what is BPD in pregnancy, let us understand how it is measured. Most of the time, the BPD is measured during a normal ultrasound during pregnancy. Most people have between one and three ultrasounds, also called sonograms, from the beginning of their pregnancy until about week 20. People who are thought to be at high risk may need more ultrasounds.
A BPD means in pregnancy is helpful in addition to the following three measurements:
Together, these three measurements help estimate the weight and age of the fetus (how far along the pregnancy is). The BPD measurement also lets the pregnant woman and their doctor know how the baby's brain is growing as it grows. The doctor is looking for the BPD measurement and the other measurements to be within what is considered the normal range.
Measuring the biparietal diameter late in pregnancy isn't as reliable for figuring out how far along the pregnancy is. Between week 12 and week 26 of pregnancy, BPD is usually accurate within 10 to 11 days for figuring out the gestational age. After week 26, though, it could be wrong by up to three weeks. Some studies also show that after week 20, BPD doesn't work as well.
The BPD in pregnancy is best assessed along an axial plane that goes across the thalami and cavum septum pellucidum. The calvarium and cerebral hemispheres should seem similar when the transducer is placed perpendicular to the skull's central axis.
Calipers need to be positioned at the outer edge of the near calvarial wall and the inner edge of the far calvarial wall. Moreover, the cerebellar hemispheres shouldn't be in the plane of the image.
If the baby's results are outside of what is considered normal, the doctor may suggest more tests. For example, if the baby's BPD is smaller than usual, that could mean that the baby's growth is restricted in the womb or that the baby's head is flatter than usual. If the baby's BPD is bigger than expected, it could mean that something is wrong with their health, like gestational diabetes.
A low BPD in pregnancy can be a sign that the head growth of the foetus needs to be watched. Women who may have been exposed to the Zika virus may worry about having a baby with a small head. If the BPD is two standard deviations below the mean, the head is too flat and microcephaly is thought to be a possibility. There are other signs of microcephaly, like how the head looks and how big it is.
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The biparietal diameter is a useful tool to measure the progress of the baby's development, and it can also help identify if a baby may have certain conditions like microcephaly or hydrocephalus. But before anyone freaks out and worries that their baby has one of these conditions, know that as long as the BPD in pregnancy looks within normal parameters, the baby is perfectly healthy.
1. Göttlicher S, Madjaric J, Krone HA. (1976). Biparietal diameter of the fetal head during pregnancy. A comparative study. NCBI
2. Lee W, Balasubramaniam M, Deter RL, Hassan SS, Gotsch F, Kusanovic JP, Gonçalves LF, Romero R. (2009). Fetal growth parameters and birth weight: their relationship to neonatal body composition. Ultrasound Obstet Gynecol.
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