Is your bump on track?

McDonald's rule of thumb says that from about week 16 to week 36, fundal height in centimeters roughly matches gestational age in weeks — at week 24, expect a fundal height around 24 cm; at week 32, around 32 cm. This calculator's week-by-week figures aren't a single expected number, though: they're 10th, 50th, and 90th percentile fundal heights for each week, drawn from Jelks 2007 (Obstet Gynecol), which is why picking a week above shows a range alongside the expected figure.

The 10th–90th percentile band is what counts as the normal range: most measurements fall somewhere inside it at a given week, with the 50th percentile representing the typical, or median, measurement — that's the figure this calculator shows as "expected." Falling inside the 10th–90th band is routine. Falling outside it — either smaller or larger — is one signal among several that a provider may want a closer look, not a diagnosis on its own.

Fundal height is a screening tool, not a diagnostic test, and inter-rater variability between providers is real — different hands measure differently. A measurement smaller than the 10th percentile ("fundal height lag," or size less than dates) is one of several reasons providers may order a growth ultrasound; it doesn't mean something is wrong. A measurement larger than the 90th percentile (size greater than dates) can reflect baby position, polyhydramnios, or simply a larger baby, and is also usually followed up with a growth ultrasound rather than treated as a standalone finding. In both cases, the ultrasound — not the tape measurement — is considered the better data.

McDonald's rule is decent in the second trimester and starts diverging after week 36 as the baby drops. Fundal height isn't typically measured before week 16, which is why this calculator covers weeks 16 through 40. Given normal measurement variability between providers, expect roughly ±2 cm of uncertainty at any given week — see the accuracy notes below for more on that confidence rating. Source: Jelks 2007, Obstet Gynecol; ACOG Practice Bulletin 134.