Fundal Height to Gestational Age Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026A quick screening check at prenatal visits
McDonald's Rule is a long-standing clinical convention applied roughly between 20 and 36 weeks gestation, where fundal height measured in centimeters approximately equals gestational age in weeks.
Worked example
For a fundal height of 28cm:
Estimated GA = ~28.0 weeks (range 26.0-30.0)
Frequently asked questions
How accurate is this rule? It is a rough screening tool, not a precise dating method - normal variation of about plus or minus 2 weeks is expected, and ultrasound-based measurements are significantly more accurate whenever precise dating is needed.
What does a measurement outside the expected range mean? A fundal height notably larger or smaller than expected for gestational age warrants further evaluation by a healthcare provider, since it can reflect factors like fetal growth patterns, amniotic fluid levels, fetal position, or dating discrepancies rather than necessarily indicating a problem on its own.