Enter a biparietal diameter (BPD) in millimetres. The tool estimates gestational age, shows the percentile, and lists a typical BPD range for that age.
* Without a known age, the percentile is not a separate check of fetal growth. This simplified reference is not a diagnostic growth chart.
How to use this calculator
- Measure BPD: use the standard transthalamic plane with calipers outer-to-inner.
- Enter BPD (mm): type the value to one decimal place if available.
- Calculate: see the estimated gestational age, BPD percentile at that age, and the typical BPD band.
- 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.
Calculator updated: September 13, 2026
Methodology and sources
Gestational age is estimated by matching BPD to a simplified smoothed 12–40-week reference at 0.1-week steps. Percentile uses the normal cumulative distribution, with z divided by the square root of 2 inside the error function. The approximate range is mean ±1.645 standard deviations. If you enter a known age, only the percentile and comparison range use it; the BPD-derived age stays visible. Outside the 21–96 mm age curve, the closest endpoint is clearly identified. The linked ISUOG guidance concerns measurement technique; this simplified reference is not a validated substitute for the chart on your ultrasound report.
What this tool calculates
- Estimated gestational age from a single biparietal diameter measurement in millimetres.
- A percentile for BPD at the known age you enter, or the BPD-derived age if none is entered, using a simplified normal model.
- A typical BPD range at the same comparison age, 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
Calculator updated: September 13, 2026
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 compares BPD with a simplified normal-distribution reference at the known age you enter. If age is inferred from the same BPD, a percentile near 50 is expected; this does not independently assess growth.
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.