Understanding fundal height results
Fundal height, also called symphysis-fundal height (SFH), is the distance in centimeters from the top of the pubic symphysis to the top of the uterus (the fundus), measured with a tape measure at prenatal visits from roughly 20 weeks of pregnancy onward. It is a simple, low-cost screening measurement for fetal growth, not a diagnostic test on its own.
The cm-equals-weeks rule
Between about 20 and 36 weeks of gestation, fundal height in centimeters approximately tracks gestational age in weeks: a pregnancy at 28 weeks typically has a fundal height near 28 cm. A measurement within about 2 cm of gestational age is considered consistent with dates; a gap of 2-3 cm is a signal to keep monitoring; a gap beyond 3 cm is usually followed up with an ultrasound to check growth, fluid volume, and dating accuracy.
McDonald's Rule
McDonald's Rule converts a fundal height measurement into an estimated gestational age using GA (weeks) = fundal height (cm) × 8/7. It is a bedside cross-check, not a replacement for dating by last menstrual period or a first-trimester ultrasound.
Johnson's formula for estimated fetal weight
Johnson's formula gives a rough third-trimester weight estimate: EFW (grams) = (fundal height (cm) − n) × 155, where n is 12 if the fetal head is still above the ischial spines (not engaged) or 11 if it has descended to or below the ischial spines (engaged). It is far less precise than an ultrasound-based estimate and works best close to term with the fetus head-down.
What can shift the measurement
Maternal body size, uterine fibroids, multiple gestation, fetal position, amniotic fluid volume, and even a full bladder can all move the measured fundal height without reflecting a true growth problem. A single discordant reading is far less informative than a trend across several visits.
When to consult a professional
This calculator performs the standard fundal-height arithmetic for reference only. It does not diagnose fetal growth restriction, macrosomia, or any pregnancy complication. Any fundal height outside the expected range should be discussed with the treating obstetric provider, who can order further assessment and interpret it alongside the full clinical picture.