BPD to Gestational Age Calculator: Percentile and Typical Range

Enter a biparietal diameter (BPD) in millimetres. The tool estimates gestational age, shows the percentile, and lists a typical BPD range for that age.

Biparietal diameter (BPD) in millimeters
Standard transthalamic plane (thalamus visible, cavum septi pellucidi, calipers outer-to-inner).
Clinician tools
Estimated GA: — — weeks
Percentile for BPD at this GA: —
Typical BPD range at this GA: —
These are broad reference values (approximate). Ultrasound assessment should be interpreted by a clinician.

What this tool calculates

  • Estimated gestational age from a single biparietal diameter measurement in millimetres.
  • A percentile for that BPD at the estimated GA using a normal model.
  • A typical BPD range at the estimated GA, shown as an approximate 5th to 95th percentile band.

Measurement plane and calipers

  • Transthalamic plane with thalami and cavum septi pellucidi visible, midline falx centred.
  • Calipers placed outer edge to inner edge of the parietal bones (leading edge method).

Computation

  • Reference table: mean and standard deviation for BPD by completed week 12 to 40. Values are smoothed and then linearly interpolated at 0.1 week steps.
  • GA estimate: invert the smoothed mean curve, selecting the GA whose mean BPD is nearest to the entered BPD.
  • Percentile: z = (BPD − mean) divided by SD at that GA. Percentile = normal CDF(z) × 100.
  • Typical range: mean ± 1.645 SD, which approximates the 5th to 95th percentile.

Scope and limits

  • Single-parameter estimates are coarse. Clinicians combine multiple biometric measures and clinical dates.
  • Fetal head shape, presentation, and measurement error affect BPD. Use consistent technique.
  • This tool is for education and counselling support. It does not replace ultrasound reporting by a clinician.

Sources

  • ISUOG Practice Guidelines, performance of the routine mid-trimester fetal ultrasound scan. Describes the transthalamic plane and BPD caliper placement. onlinelibrary.wiley.com

Last updated: 6 Oct 2025

How to use this calculator

  1. Measure BPD: use the standard transthalamic plane with calipers outer-to-inner.
  2. Enter BPD (mm): type the value to one decimal place if available.
  3. Calculate: see the estimated gestational age, BPD percentile at that age, and the typical BPD band.
  4. Compare or repeat: re-enter a new BPD to update the estimate; use the Ultrasound Due Date tool for date-based estimates.

Percentiles are approximate and depend on technique, fetus position, and equipment.

Last updated: October 2025

Frequently asked questions

What is BPD?

Biparietal diameter is the transverse fetal head measurement taken on the transthalamic plane. It is reported in millimetres.

How does the tool estimate gestational age?

It matches your BPD to a smoothed mean BPD curve versus gestational age and returns the closest age at 0.1-week steps.

What does the percentile mean?

The percentile is the position of your BPD relative to peers at the estimated age using a normal model. It is a guide, not a diagnosis.

Why might my result differ from the ultrasound report?

Different charts, machine presets, and operator technique can shift values. Reports also integrate multiple measurements and clinical context.

Can I use this to set my due date?

Use ultrasound-based dating with CRL in the first trimester where available. For date estimates, see the Ultrasound Due Date calculator.