The formula
height in cm ≈ weeks of gestation ± 2
Where it comes from
It is measured with a tape from the top of the pubic bone to the top of the womb, and the match with the weeks is one of biology's useful coincidences: the womb grows about a centimetre a week over that stretch. Before week 20 the womb is still low and the measurement is not reliable; after week 36 the baby settles into the pelvis and the height stops rising, or even drops.
How to work it out by hand
- Check that you are between weeks 20 and 36
- The expected height in centimetres is the number of weeks
- The normal margin is two centimetres either way
- Compare your measurement against that interval
What is worth knowing
A measurement above can come from a large baby, extra amniotic fluid, a multiple pregnancy or simply the mother's build; one below, from slow growth, low fluid or a different position of the baby. Technique matters too, and so does a full bladder, which can add several centimetres. That is why a single measurement matters less than the curve: two checks in a row with no increase say more than one number outside the range.