This article was consulted by an Obstetrics and Gynecology specialist at Vinmec Hai Phong International General Hospital.
Biparietal Diameter (BPD) is a crucial measurement included in fetal ultrasound. Through this measurement, doctors can assess the baby’s development and estimate the baby’s weight and size at birth.
1. What is the Biparietal Diameter (BPD)?
The Biparietal Diameter (BPD) is the diameter measured at the widest section of the fetal skull (from the forehead to the back of the head). It is considered as the diameter of the baby’s head.
Note that BPD is entirely different from the fetal head circumference. Head circumference measures the perimeter around the baby’s head. Biparietal Diameter measures the width of the baby’s head. These two values are related in the same way as the circumference and diameter of a circle.
In fetal ultrasound, BPD is used to estimate fetal weight, determine gestational age, and assess the growth rate of the fetus.
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2. When can BPD be measured through ultrasound?
The BPD measurement can be taken via ultrasound starting from 13 weeks of pregnancy and is most accurate between 13 and 20 weeks. During this time, the fetal head develops rapidly. Measuring BPD after this period may provide less accurate results.
3. What is the normal range for Biparietal Diameter (BPD)?
The average BPD of a fetus between 13 and 40 weeks of pregnancy is typically 88–100mm, with an average of approximately 94mm.
If the BPD is smaller than normal, it may indicate delayed fetal growth or that the baby has a flatter skull shape compared to others. In contrast, if the BPD is larger than normal, it may suggest that the baby has a larger-than-average head size, which could pose challenges for vaginal delivery, especially for first-delivery mothers. In some cases, an abnormally high BPD, combined with other parameters exceeding normal ranges, may be linked to maternal gestational diabetes. If the baby is too large, doctors may recommend a cesarean section for safety.
When a fetus’s BPD is not within the standard range, doctors may request a repeat ultrasound or further testing (e.g., amniocentesis to check DNA) to confirm the baby’s health status. However, doctors also consider other measurements such as femur length, abdominal circumference, and head circumference to provide a comprehensive assessment of the baby’s growth, particularly brain development.
4. BPD Chart
| Gestational Age (Weeks) | Biparietal Diameter (mm) | Gestational Age (Weeks) | Biparietal Diameter (mm) |
| 13 | 21 | 27 | 68 |
| 14 | 25 | 28 | 71 |
| 15 | 29 | 29 | 73 |
| 16 | 32 | 30 | 76 |
| 17 | 36 | 31 | 78 |
| 18 | 39 | 32 | 81 |
| 19 | 43 | 33 | 83 |
| 20 | 46 | 34 | 85 |
| 21 | 50 | 35 | 87 |
| 22 | 53 | 36 | 89 |
| 23 | 56 | 37 | 90 |
| 24 | 59 | 38 | 92 |
| 25 | 62 | 39 | 93 |
| 26 | 65 | 40 | 94 |
5. How to Calculate Fetal Weight Based on Biparietal Diameter?
Based on BPD, fetal weight can be estimated using the 2 following formulas:
- Weight (grams) = [BPD (mm) – 60] x 100
- Example: If BPD = 90mm, then fetal weight = (90 – 60) x 100 = 3kg.
- Weight (grams) = 88.69 x BPD (mm) – 5062
- Example: If BPD = 90mm, then fetal weight = (88.69 x 90) – 5062 = 2920g
6. How to Ensure Normal BPD Development?
- During pregnancy, mothers should maintain a balanced diet that includes four essential nutrients: protein, carbohydrates, fats, and vitamins (from fruits and vegetables). Especially, It’s particularly important to focus on increasing protein intake to support fetal development. For example, shrimp and crab are rich in calcium and protein, or you can include fish, beef, chicken, and eggs in your diet.
- Attend regular prenatal check-ups to monitor fetal growth.
- Calcium supplementation: prevents osteoporosis in the mother and ensures the baby receives enough calcium for bone and height development.
- Prenatal vitamins should be taken to maintain the mother's overall health.
- Tetanus vaccination should be administered to both mother and baby once the pregnancy reaches 20 weeks. Two doses are required: the second dose should be given at least one month after the first, and the second dose must be administered at least one month before delivery.
The biparietal diameter is an essential parameter for determining gestational age, estimating fetal weight, and assessing brain development. Pregnant women should attend regular prenatal check-ups to keep track of their baby’s BPD and ensure early detection of any abnormalities.
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