Fundal height is one of the simplest, lowest-cost screening checks in prenatal care — a tape-measure reading compared against a rule of thumb. This calculator applies McDonald's rule to flag when a measurement falls outside the normal range so you understand what a provider's note or a flagged visit actually means.
How the Fundal Height Calculator works
The calculator applies McDonald's rule: between about 20 and 36 weeks of pregnancy, fundal height in centimeters is expected to roughly equal gestational age in weeks. It compares your measured fundal height to that expected value and reports the difference (delta) in centimeters. A delta within +/-2 cm is classified as within the normal range; 2 to 3 cm is borderline; more than 3 cm is flagged as a discrepancy warranting further evaluation. Outside the 20-36 week reliability window, the same math still runs but the estimate is less accurate, and the calculator notes that ultrasound dating is preferred.
Inputs and what they mean
Gestational age (weeks) should come from your provider's dating — typically a first-trimester ultrasound or last menstrual period — not a guess. Measured fundal height (cm) is read directly from a tape measure placed from the pubic symphysis to the top of the uterine fundus, normally recorded by a clinician at each prenatal visit. Both inputs need to be accurate for the delta to be meaningful; an inaccurate gestational age is itself one of the most common reasons for an apparent fundal height 'discrepancy'.
Limits and edge cases
Fundal height is a screening heuristic, not a precise measurement — technique variability, maternal body size, uterine fibroids, fetal position, a full bladder, and multiple gestation (twins) can all shift the reading without reflecting an actual growth problem. The rule is also less reliable before 20 weeks or after 36 weeks. A single flagged measurement is not a diagnosis; it is a prompt for follow-up, most commonly an ultrasound to directly assess fetal growth, amniotic fluid, and dating. Always discuss a flagged result with your obstetric provider rather than relying on this tool alone.