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.)