Fundal Height Defined
Fundal height, or McDonald's Rule, is a measure of the size of the uterus to assess fetal growth and development. It is measured from the top of the pubic bone to the top of the uterus in centimeters and it should match your baby's gestational age, within one to three centimeters; for example, a measure of 22 centimeters should be seen on a 19 to 25 weeks pregnant woman. This measurement, as you might suspect, should increase as your pregnancy continues toward a due date.
How It Is Used
The fundal height is diagnostic tool held over from the days before ultrasound, but it is still useful in large clinics where a patient may not get the same doctor twice in a row for her prenatal visits.
There is no measurement more exact than ultrasound (and even this isn't perfect), but the fundal height, although often recorded with each visit, can be even more inaccurate. Many obstetricians in private practice, who see the same patients repeatedly, don't even record fundal height but merely note whether the size of the uterus is compatible with the gestational age. A physician who is familiar with his or her patients will generally know when a baby isn't growing right. And if there is suspicion of a growth restricted baby or an LGA (large for gestational age) baby, then ultrasound is usually the best way to sort out any worries—and not fundal height.
But the fundal height is very useful in the larger clinics, because many different doctors will see a patient before her due date. Because there needs to be at least some objective frame of reference among the different doctors, the fundal height measurement can still be helpful.
Like no two scales read the same weight the same, no two doctors are going to measure fundal height the same. But if a doctor gets a discrepancy that's wildly different from what was expected based on the previous doctor's fundal height last visit, then ultrasound is justified. This is not very exact science, but it works remarkably well in the larger clinics.