What this calculator does
McDonald's rule is one of the neatest coincidences in obstetrics: between roughly 20 and 36 weeks, the distance in centimetres from the pubic bone to the top of the uterus is approximately the gestational age in weeks.
It is a screening check, not a measurement. A tolerance of about 3 cm either way is considered normal, so a fundal height of 24 cm at 28 weeks by dates falls outside that range and is a prompt to investigate rather than a finding in itself.
The formula
Fundal height in centimetres is read directly as gestational age in weeks and compared against the age calculated from dates. The difference is checked against the conventional 3 cm tolerance.
| Term | Meaning |
|---|---|
| Fundal height | The distance from the pubic symphysis to the top of the uterus, in centimetres. |
| McDonald's rule | The observation that this distance approximates gestational age in weeks. |
| Tolerance | The conventional 3 cm margin either side, beyond which further assessment is usual. |
The inputs explained
| Field | What to enter |
|---|---|
| Fundal height (cm) | Fundal height in centimetres, measured with a tape from the pubic bone to the top of the uterus. |
| Gestational age by dates (weeks) | Gestational age in weeks as calculated from dates or a dating scan. |
When to use it
Routine antenatal screening
Fundal height is measured at most appointments from around 24 weeks as a growth check.
Understanding a referral for a scan
A measurement outside the tolerance usually prompts an ultrasound rather than a conclusion.
Following growth across appointments
The trend over several measurements matters more than any single one.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How is each measurement read against dates?
Three fundal height measurements at the same gestational age.
| Fundal height | Difference | Reading |
|---|---|---|
| 24 cm | -4.0 cm/weeks | Outside the usual ±3 cm tolerance: worth discussing with a clinician |
| 28 cm | +0.0 cm/weeks | Within the usual ±3 cm tolerance of McDonald’s rule |
| 32 cm | +4.0 cm/weeks | Outside the usual ±3 cm tolerance: worth discussing with a clinician |
Questions
How reliable is fundal height?
Only moderately. It is affected by maternal body size, fibroids, bladder fullness, fetal position and amniotic fluid volume, and measurement varies between observers. It is used because it is quick and free, not because it is accurate.
Why does it only work from 20 to 36 weeks?
Before about 20 weeks the uterus is too low to measure reliably, and after about 36 weeks the baby often descends into the pelvis, which reduces fundal height even as the pregnancy advances.
What happens if the measurement is off?
An ultrasound is usually arranged to assess growth and fluid directly. A single measurement outside the range has many benign explanations, and the scan is what distinguishes them.
Can I measure this at home?
The technique matters a great deal and measurements vary substantially between people, so a home figure is unlikely to be meaningful. Antenatal appointments exist to do this consistently.
For dating a pregnancy from an early scan, see the crown-rump length calculator. For weight gain guidance, see the pregnancy weight gain calculator.