Biparietal Diameter (BPD) in Pregnancy: What You Need to Know

Ultrasound scans do more than show images of your baby. They provide doctors with important measurements that help track growth and development. One of the key numbers often mentioned in fetal biometry is the Biparietal Diameter (BPD), which measures the width of your baby’s head.

The term may sound clinical, but the purpose is simple. BPD helps estimate your baby’s age, monitor growth, and check that development is on track.

It is natural for medical terms and figures to feel overwhelming during checkups. Understanding BPD can make appointments less stressful and help you feel more confident when discussing results with your doctor. In this guide, we will explain what BPD is, why it matters, when it is measured, and how it supports a healthy pregnancy.

What Is Biparietal Diameter in Pregnancy?

The biparietal diameter is one of the standard measurements taken during a fetal ultrasound. It refers to the distance across the widest part of your baby’s head, measured from one parietal bone (the large, curved bone forming the upper sides of the skull) to the other. In simple terms, it represents the width of the fetal skull.

BPD is most commonly measured during the second trimester and serves as a key indicator of your baby’s growth and development. Doctors use it to estimate gestational age, track growth patterns, identify possible concerns, and even calculate an approximate fetal weight.

It is important to understand that BPD is not the same as head circumference (HC). Head circumference measures the distance around the baby’s skull, while BPD focuses only on the straight-line width. The two are related much like the diameter and circumference of a circle.

READ: What is low birth weight? Causes & Diagnosis Methods

When Can BPD Measurement Be Done Through Ultrasound?

BPD is usually measured during the second trimester, between 13 and 28 weeks of pregnancy. During this time, the fetal head grows rapidly, which makes it easier to get an accurate measurement. Later in pregnancy, it can be harder to capture a clear image.

Sometimes, BPD is also measured in the third trimester to check on growth or the baby’s well-being. However, these later measurements are generally less accurate.

Why Is BPD Measurement Important in Pregnancy?

Measuring the biparietal diameter helps monitor your baby’s development:

• Estimates gestational age and helps predict the due date
• Tracks fetal head growth over time to ensure it is steady and healthy
• Detects possible issues, such as growth restriction or conditions like hydrocephalus (excess fluid in the brain)
• Allows doctors to identify concerns early and provide appropriate care

How Is BPD Measured?

BPD is measured during a standard transabdominal ultrasound. The sonographer starts by locating the baby’s head and positioning the probe to obtain a clear cross-sectional image at the level of the thalami and the cavum septum pellucidum.

Once the correct view is achieved, callipers are placed at the widest part of the skull, perpendicular to the midline. The measurement is taken from the outer edge of the skull closest to the probe to the inner edge on the opposite side. The result is recorded in millimetres.

Accurate positioning is essential for a reliable measurement. Several factors can affect BPD measurement during an ultrasound:

• Baby’s position can make imaging difficult
• A tilted uterus may affect the view
• Amniotic fluid levels can interfere
• Very early or late stages of pregnancy may be challenging
• Maternal obesity, fetal movements, and ultrasound limitations can impact accuracy
• Growth in twins and triplets may slow in later pregnancy
• Head shape differences, such as dolicocephaly, can affect BPD

What Is the Normal Range for Biparietal Diameter?

The normal BPD depends on the baby’s gestational age. While exact numbers can vary, typical ranges for a single baby are approximately:

• 13 weeks - 21-28 mm
• 20 weeks - 40-58 mm
• 28 weeks - 65-81 mm
• 40 weeks - 86-102 mm

A BPD measurement within the expected range generally indicates that the baby’s head is growing normally. Small differences are common and not usually a cause for concern.

Measurements that are significantly higher or lower than the average may require closer monitoring or additional tests, such as a repeat ultrasound or amniocentesis, based on your doctor’s recommendation.

• A smaller BPD may indicate slower growth or a flatter head shape.
• A larger BPD may signal a bigger head, which could complicate normal vaginal delivery or lead to a recommended cesarean.
• A very high BPD, combined with other measurements, can be linked to gestational diabetes.

READ: Newborn Care Importance

Biparietal Diameter Chart

Here’s a BPD fetal measurement chart for different stages of pregnancy:

Remember, every pregnancy is different, and your doctor will look at other growth measurements to check your baby’s overall health and see if the numbers are normal for your pregnancy.

How to Support Normal BPD Development

• Eat a balanced diet that includes protein, carbohydrates, fats, and vitamins from fruits and vegetables. Add protein-rich foods such as poultry, meat, and seafood to support fetal growth.
• Take calcium supplements and prenatal vitamins to support your bones, aid your baby’s bone and height development, and maintain overall health during pregnancy.
• Get the tetanus vaccine after 20 weeks of pregnancy. Two doses are required, given at least one month apart, with the second dose completed at least one month before delivery.
• Attend regular prenatal check-ups to track your baby’s development.

The Bottom Line

BPD is an important ultrasound measurement used to monitor your baby’s growth. Small variations are usually nothing to worry about, but results that fall well outside the expected range may require closer follow-up with other checks. Keep in mind, BPD is just one piece of the overall picture, and your doctor will guide you if any further tests are needed.