Fundal Height Calculator — McDonald's Rule
Use McDonald's rule to estimate expected fundal height from gestational age, or work backwards from a measured fundal height to estimate gestational age. Most reliable between 20 and 36 weeks of pregnancy.
Calculate
weeks
Normal range: 26–30 cm (±2 cm from expected)
- 1
Gestational age (input)
28McDonald's rule: fundal height in cm ≈ gestational age in weeks. - 2
Expected fundal height
1 cm × 28 weeks = 28
How does this calculator work?
McDonald's rule: fundal height in cm ≈ gestational age in weeks, with a normal range of ± 2 cm, reliable from 20 to 36 weeks. A reading outside ± 2 cm of the expected value is a screening signal for further assessment — it does not diagnose a problem on its own. Always discuss measurements with your midwife or doctor.
Formula
How this is calculated
Fundal height (also called symphysis-fundal height, SFH) is the distance in centimetres from the top of the pubic symphysis to the top of the uterine fundus, measured with a tape measure at a prenatal visit. McDonald's rule, established by John McDonald in 1906 and still used in routine antenatal care, states that between about 20 and 36 weeks of pregnancy this distance in centimetres approximately equals the gestational age in weeks: a 28-week pregnancy should measure approximately 28 cm, ± 2 cm.
The calculator works in both directions. Given a gestational age it shows the expected fundal height and the normal range of ± 2 cm. Given a measured fundal height it estimates the gestational age in weeks ± 2 weeks. Measurements more than 2 cm smaller than expected ("small for dates") may prompt further assessment for fetal growth restriction or oligohydramnios; measurements more than 2 cm larger ("large for dates") may indicate macrosomia, multiple pregnancy or polyhydramnios.
The rule is least reliable before 20 weeks (the uterus has not yet reached the umbilicus) and after 36 weeks (the fetal head may engage, reducing the apparent height). Maternal factors such as obesity, uterine fibroids and full bladder can affect the measurement. A fundal height outside the normal range is a screening finding, not a diagnosis, and always requires clinical interpretation and often ultrasound confirmation.
Frequently asked questions
Between 20 and 36 weeks, a normal fundal height is within ± 2 cm of the gestational age in weeks. For example, at 30 weeks a normal measurement is 28–32 cm. Outside this range the midwife or doctor will assess further, often with an ultrasound.
No. Fundal height is a quick, low-cost screening tool used at routine antenatal visits. It cannot measure fetal weight, position or organ structure. An ultrasound is required for detailed assessment of fetal growth and wellbeing.
After about 36 weeks the fetal head often descends into the pelvis in preparation for birth (engagement). This can cause the fundal height to plateau or even decrease, so the 1 cm per week rule becomes unreliable and ultrasound biometry is preferred for growth monitoring.
Also known as
TG we-Calculate Editorial Team. (2026). Fundal Height Calculator — McDonald's Rule [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/fundal-height-calculator
TG we-Calculate Editorial Team. "Fundal Height Calculator — McDonald's Rule." TG we-Calculate. 2026. https://we-calculate.com/calculator/fundal-height-calculator.
TG we-Calculate Editorial Team, "Fundal Height Calculator — McDonald's Rule," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/fundal-height-calculator
@misc{wecalculate_fundal_height_calculator, title = {Fundal Height Calculator — McDonald's Rule}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/fundal-height-calculator}}, year = {2026}, note = {TG we-Calculate} }
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