Biparietal Diameter Age Calculator
Biparietal diameter (BPD) is the width of the fetal head measured across the skull at the level of the thalami. It is a core biometry parameter for estimating gestational age in the second and third trimesters. This calculator applies the Hadlock regression equation to convert a BPD in centimetres into an estimated gestational age in weeks. BPD dating is most accurate in the second trimester; later, clinicians combine it with head circumference, abdominal circumference, and femur length. Results here are for education only and must be confirmed by a qualified clinician.
Hadlock BPD formula
GA (weeks) = 9.54 + 1.482 * BPD + 0.1676 * BPD^2
BPD is in centimetres
Days = weeks * 7
Weeks + days = floor(weeks) w, remainder d
This quadratic regression relates head width to gestational age. It is best applied in the second trimester, where head growth is rapid and predictable, before normal biological variation widens later in pregnancy.
BPD biometry context
- BPD is measured at the level of the thalami in a true transverse plane.
- Second-trimester BPD dating is accurate to about 7 to 10 days.
- Accuracy declines in the third trimester as growth varies more.
- Head shape (dolichocephaly or brachycephaly) can affect BPD, so head circumference is often preferred.
- Multiple parameters together give the most reliable estimate of age and weight.
Biparietal diameter: frequently asked questions
What is biparietal diameter?
Biparietal diameter (BPD) is the ultrasound measurement of the width of the fetal head, from one parietal bone to the other, taken at the level of the thalami. It is a core second- and third-trimester biometry parameter used to estimate gestational age and fetal size.
Which equation does this calculator use?
It uses the Hadlock regression for gestational age from BPD: gestational age in weeks = 9.54 + 1.482 times BPD + 0.1676 times BPD squared, where BPD is in centimetres. This is a standard published reference equation.
How accurate is BPD dating?
BPD dating is most reliable in the second trimester, roughly 14 to 20 weeks, where it is accurate to within about 7 to 10 days. As pregnancy advances, normal growth variation widens, so a single BPD reading becomes a less precise estimate of age.
Why use multiple biometry parameters?
Clinicians usually combine BPD with head circumference, abdominal circumference, and femur length to estimate gestational age and fetal weight. Using several parameters reduces the error from any single measurement and accounts for variations in head shape.
Does this replace clinical assessment?
No. This calculator is for general education. Gestational age, fetal growth, and due dates should be determined by a qualified sonographer or clinician using calibrated equipment and full clinical context.
Official sources
- U.S. National Library of Medicine, StatPearls: Gestational Age Assessment.
- American College of Obstetricians and Gynecologists: Clinical guidance.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.