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.
Discrepancies to Consider
Relying on fundal height as above must take into account explainable discrepancies.
- A baby who has “dropped” between the last visit and the current one may show a fundal height less than last visit. A simple pelvic exam can determine the descent of the baby’s head into the maternal pelvis to explain this.
- If a baby is turned sideways, as often occurs in mid-pregnancy, the fundal height can be unusually short for what’s expected, because the greatest dimension lies across the horizontal.
- A breech baby, usually sitting up high in the uterus, will yield an unusually large fundal height.
- Twins (or more!) will give a fundal height way too big for the gestational age.
These are all circumstances that can defuse concerns over abnormal fundal heights.
The fundal height can also be out of sync with what’s expected for the gestational age due to abnormal conditions:
- Oligohydramnios—too little fluid, taking away from the entire mass effect, leading to a smaller fundal height.
- Hydramnios and polyhydramnios—too much fluid, indicating possibly genetic problems or anatomical problems with the baby.
- Abnormal position of the baby close to term. A sideways baby is forgivable in mid-pregnancy, but the later into the pregnancy the unusual position remains, the more likely there could be an abnormality.
How You Can Measure Fundal Height
The trick I use is to judge the height of the fundus in relation to the umbilicus (belly button). Generally, the pregnancy is halfway (about 20 weeks) when the fundal height is palpable (felt) at the umbilicus. For every finger’s width above your belly button the fundal height reaches, you can add a week. Likewise, below the umbilicus, you can subtract a week. (But this relationship is only applicable between 15 and 25 weeks. Before and after that this relationship goes out the window.)