How this calculator works
The INTERGROWTH-21st study provides a median and standard deviation for symphysis-fundal height in healthy singleton pregnancies. This page limits the forward comparison to 16 through 40 exact weeks. A separate published regression estimates age from height when more reliable dating is unavailable; it is not the inverse of the median-height equation. Height is a screening measure affected by technique and other factors. The table provides the reference at whole weeks in place of a growth chart. This tool does not estimate fetal weight or diagnose abnormal growth. The result is an estimate for information only and is not medical advice.
Formula
With GA in decimal weeks and SFH in cm: median = 5.133374 + 0.1058353119 GA² - 0.0231295 GA² ln(GA); SD = 0.9922667 + 0.0258087 GA; z = (measured - median)/SD. Age regression = 6.585838 - 2.7072585 sqrt(SFH) + 1.295291 SFH.
Worked example
At exactly 20 weeks the expected median is about 19.75 cm and SD 1.51 cm. A 20 cm measurement gives a z-score near 0.16.
Advertisement
Frequently asked questions
Can this replace a dating scan?
No. Use the established clinical due date when available; height-based dating is much less precise.
Why is fetal weight not shown?
The cited INTERGROWTH height model does not provide an estimated fetal weight equation.
Does a z-score diagnose a small fetus?
No. It can prompt professional review and ultrasound, but is not a diagnosis.