BPD in Pregnancy: A Key Factor in Estimating Fetal Weight and Gestational Age

TL;DR

Biparietal diameter (BPD) is the measurement of your baby's head width from one side to the other, taken during a pregnancy ultrasound (Radiopaedia). It is one of four key measurements (along with head circumference, abdominal circumference and femur length) used to estimate your baby's gestational age and weight, and to check that the head and brain are growing normally. BPD is most accurate for dating between about 13 and 20 weeks, and less reliable later in pregnancy. A reading a little outside the "normal" range is usually not a problem on its own, doctors look at all the measurements together and at the growth trend over time. Only your doctor can interpret your scan, so use the week-by-week table here as a general guide, not a diagnosis.

Quick Answer

BPD (biparietal diameter) is the width of your baby's head measured on ultrasound, from one parietal bone to the other. It helps estimate how far along your pregnancy is and your baby's weight, and gives clues about head and brain growth. It is most accurate between about 13 and 20 weeks. Normal BPD rises steadily with each week (roughly 2.9 cm at 14 weeks to about 9.5 cm near term). A high or low BPD is not automatically a problem, it is interpreted alongside the other scan measurements and your baby's growth over time. If your BPD is outside the expected range, your doctor may simply repeat the scan or do further checks.

Author: Mylo Editorial Team, Mylo Parenting Desk Medically reviewed by: Mylo Editorial Board, aligned with FOGSI and radiology imaging guidance Last updated: 13 July 2026

Medical Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice. Ultrasound measurements must be interpreted by your doctor or radiologist in the context of your full scan and history. Do not draw conclusions from a single number, always discuss your results with your doctor.

Key Takeaways

• BPD is the width of your baby's head, measured on ultrasound (Radiopaedia)
• It helps estimate gestational age and, with other measurements, fetal weight
• It is most accurate for dating between about 13 and 20 weeks
• BPD is used together with head circumference, abdominal circumference and femur length
• Normal BPD increases steadily each week (about 2.9 cm at 14 weeks to ~9.5 cm near term)
• A single reading slightly outside range is usually not a concern on its own
• Doctors look at all measurements together and at the growth trend over time
• Only your doctor can interpret your scan, so avoid self-diagnosing from one number

What Does BPD Mean in Pregnancy?

Biparietal diameter (BPD) is one of several measurements taken during a pregnancy ultrasound. It measures how wide your baby's head is, from one parietal bone (on one side of the skull) to the other (Radiopaedia).

The skull has two parietal bones, one on the left and one on the right. To picture the BPD, imagine a line running from just above one ear, across the top of the head, to just above the other ear. Together with the head circumference (HC), abdominal circumference (AC) and femur length (FL), BPD helps your doctor estimate your baby's weight and age, and check that the head and brain are developing well.

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How Is BPD Measured?

BPD is usually measured during a routine pregnancy ultrasound (sonogram). Most women have between one and three scans through pregnancy, and those with higher-risk pregnancies may have more (Cleveland Clinic).

The sonographer measures BPD on a specific cross-section of the head (an axial view through the thalami and the cavum septum pellucidum), placing the on-screen calipers from the outer edge of the near side of the skull to the inner edge of the far side. It is measured alongside the other three parameters to estimate weight and age.

BPD is most accurate for dating early on. Between about weeks 12 and 26, it estimates gestational age to within roughly 10 to 11 days. After week 26, it becomes less reliable and can be off by up to about three weeks, because babies grow at different rates later in pregnancy. This is why early scans are preferred for confirming your due date (ACOG).

Normal BPD Range Week by Week

BPD increases steadily as your baby grows. The values below are approximate averages, your scan report may use a slightly different chart, and small differences are normal (StatPearls).

Remember, BPD is just one part of the picture and should always be read together with the other measurements, not on its own.

What If BPD Is Outside the Normal Range?

First, do not panic. A single BPD reading slightly above or below the range is very often completely normal, it can simply reflect your baby's build, head shape or position during the scan. Doctors interpret BPD alongside the other measurements and, importantly, look at how your baby is growing over time.

If your BPD is outside the expected range, your doctor may simply repeat the scan later to track the trend, or arrange additional tests. A measurement well below the average (around two or more standard deviations below the mean), together with other signs, may prompt assessment for microcephaly, but this is a specialist judgement, not something to conclude from one number.

You may also like: Fetal growth and development during pregnancy

Indian Context

In India, ultrasound reports full of measurements like BPD, HC, AC and FL can be confusing and cause unnecessary worry, especially when a single value is flagged. It helps to remember that your radiologist and obstetrician read these together, and that Indian babies, like all babies, vary in size. Also note that, under the PCPNDT Act, ultrasounds in India are strictly for medical assessment and never for revealing the baby's sex. If any measurement concerns you, ask your doctor to explain it in the context of your whole scan rather than worrying over one figure.

Myths vs Facts

Frequently Asked Questions

Ultrasound report mein BPD ka matlab kya hota hai? (Hinglish)

BPD (biparietal diameter) aapke bacche ke sir ki chaudai ka maap hai, ek taraf se doosri taraf tak. Ise head circumference, abdominal circumference aur femur length ke saath milakar bacche ki age aur weight ka andaaza lagaya jaata hai. Yeh sir aur brain ki growth ke baare mein bhi jaankari deta hai.

BPD normal range se thoda alag hai, kya chinta ki baat hai? (Hinglish)

Aksar nahin. Ek baar ka thoda kam ya zyada BPD normal ho sakta hai, yeh bacche ke sir ke aakaar ya position par bhi depend karta hai. Doctor saari measurements aur growth ke trend ko dekh kar batate hain. Zaroorat pade to scan dohraya ya extra test kiya jaata hai.

At what stage is BPD most accurate for dating?

BPD is most accurate for estimating gestational age before about 20 weeks, and quite reliable up to around 26 weeks. After that, babies grow at different rates, so BPD becomes less accurate for dating.

Can BPD tell my baby's weight?

Not by itself. BPD is combined with head circumference, abdominal circumference and femur length to estimate your baby's weight. All four together give a much better estimate than any single measurement.

Should I worry if my baby's BPD is high or low?

Usually not from one reading. It is interpreted with the other measurements and your baby's growth over time. Your doctor will tell you if any follow-up scan or test is needed, so always discuss the report with them.

References

1. Radiopaedia. Biparietal diameter. radiopaedia.org
2. Cleveland Clinic. Fetal Ultrasound. my.clevelandclinic.org
3. StatPearls (NCBI Bookshelf). Prenatal Ultrasound. www.ncbi.nlm.nih.gov
4. American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion). www.acog.org
5. American Institute of Ultrasound in Medicine (AIUM). Practice Parameter for the Performance of Obstetric Ultrasound Examinations. www.aium.org
6. NHS. 20-week screening scan. www.nhs.uk
7. Mayo Clinic. Fetal ultrasound. www.mayoclinic.org
8. Göttlicher S, Madjaric J, Krone HA. (1976). Biparietal diameter of the fetal head during pregnancy: a comparative study. NCBI. pubmed.ncbi.nlm.nih.gov
9. Lee W, Balasubramaniam M, Deter RL, et al. (2009). Fetal growth parameters and birth weight: their relationship to neonatal body composition. Ultrasound in Obstetrics & Gynecology. pubmed.ncbi.nlm.nih.gov