How this instrument works
McDonald's rule is a decades-old shortcut: measured in centimeters from the pubic bone to the top of the uterus, fundal height is assumed to roughly equal gestational age in weeks between about 20 and 36 weeks of pregnancy. A measurement of 24 cm reads as an estimated 24 weeks; 32 cm reads as 32 weeks. It's arithmetic-free by design — no formula beyond noticing the two figures usually sit close together — which is exactly why it survived as a bedside habit long after more precise dating tools existed.
This calculator answers a narrower question than the other dating tools on this site: given a fundal height someone already measured, what does the rule say gestational age is? It isn't projecting a due date and it isn't counting days from a last period — it's translating one physical measurement into weeks using the traditional one-to-one assumption, exactly as a clinician would jot it on a chart.
The traditional teaching allows roughly 2 to 3 cm of normal variation either side of the expected figure, but modern validation work is considerably less flattering than that folk wisdom suggests. A 2022 systematic review pooling thousands of pregnancies found this style of dating carried a mean bias of about 14 days and limits of agreement spanning roughly ±43 days against ultrasound-confirmed dates — far looser than 'one centimeter equals one week' implies. That's why obstetric guidance today treats the rule as a low-tech screen for grossly abnormal growth, flagging when a repeat scan is worth ordering, rather than as a primary way to date a pregnancy; first-trimester ultrasound holds that job now.
- Measure fundal height in centimeters, from the top of the pubic bone to the top of the uterus, with an empty bladder for consistency.
- Enter the measurement into Fundal height (cm).
- Read Estimated gestational age (weeks) — the rule simply reports the same number back as weeks.
- Treat a reading well outside the expected range as a prompt to ask about a scan, not as a diagnosis on its own.
Worked example — 24 cm fundal height
A fundal height of 24 cm is measured at a routine visit. McDonald's rule reads that directly as an estimated 24 weeks of gestation — no conversion beyond treating the centimeter figure as a week count.
At 32 cm the same reading gives 32 weeks, and at 20 cm — near the lower edge of where the rule is meant to apply — it gives 20 weeks. If any of these readings sat 4 or 5 cm off the gestational age already established by an early scan, that gap, not the fundal-height number itself, is what would prompt a closer look at fetal growth.
Questions
How accurate is McDonald's rule really?
Less accurate than the simple 'one centimeter equals one week' framing suggests. A 2022 meta-analysis pooling roughly 5,800 pregnancies found this style of dating carried a mean bias near 14 days versus ultrasound-confirmed gestational age, with limits of agreement spanning about ±43 days — wide enough that clinical guidance no longer treats it as a dating method, only as a screen for growth that looks obviously off.
Is this calculator meant to replace an ultrasound due date?
No. First-trimester ultrasound remains the most reliable way to date a pregnancy, and an established due date from an early scan should take priority over anything McDonald's rule produces. This tool reconstructs the traditional bedside estimate from a measurement, useful for understanding what a clinician's tape-measure check is doing, not for setting or revising a due date.
Why does the rule only apply between about 20 and 36 weeks?
Before roughly 20 weeks the uterus hasn't risen far enough above the pelvis for the centimeter-to-week relationship to hold, and after about 36 weeks the baby's head can drop into the pelvis, shrinking the measurement independent of continued growth. Outside that window the same-number assumption breaks down in both directions.
What does it mean if my fundal height doesn't match my dates?
On its own, not much — the traditional teaching allows a few centimeters of normal variation either side of the expected figure. A reading that lands well outside that range, or one that stalls or jumps between visits, is what typically prompts a clinician to order a scan to look at fetal growth directly, rather than relying on the tape measure alone.
Why do clinicians still use such an old, imprecise method?
Because it costs nothing, takes seconds, and needs no equipment beyond a tape measure — useful anywhere a scanner isn't sitting in the room. Its job today is narrow: flag a pattern worth a closer look, not deliver a precise gestational age. For that narrow job it remains a reasonable low-tech screen.
Who first came up with the rule?
It's generally attributed to a physician named McDonald in the early 1900s, long before controlled validation studies existed to check it. Because that original source predates modern indexed research, the honest citation for how the rule actually performs today is a recent study, not the historical name attached to it.
References
- Measurement of symphysis-fundal height for gestational age estimation in low-to-middle-income countries: a systematic review and meta-analysis — PLOS ONE, 2022
- Mayo Clinic — Fundal height: An accurate sign of fetal growth?
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.