At each prenatal visit your doctor or midwife probably measures your uterus to see how the baby is growing. In the first half of pregnancy, the size is estimated by how big the uterus feels -- 15 weeks is like a large grapefruit, give or take. After 20 weeks, the distance is measured in centimeters from the pubic bone to up over the top of the uterus.

On average this measurement is about the same number of centimeters as weeks' gestation, give or take 4.  This number gives us something to assess fetal growth by, but of course the number is also influenced by the position of the baby, how many babies are in there, the amount of amniotic fluid, and the mother's height and weight.

If you measure significantly larger or smaller than expected, ultrasound will probably be used to try to figure out why. Early in pregnancy, the most common reason for a size-dates discrepancy is that your dates are wrong and you are either further along or earlier than originally calculated. Larger size can also represent twins.

When there is a question about dates, it is really important to sort that out as early as possible, since first-trimester ultrasounds are accurate to within a week of the true gestational age, whereas second-trimester fetuses can differ by two weeks, and third-trimester by three weeks.

This is probably due to the natural differences in sizes of babies as they get close to term, while first-trimester embryos are the same size on the same day of pregnancy. Accurate due dates are important for management of the pregnancy if complications develop, for example, preterm labor or going well past the due date.

Over the next few weeks I will write more about other issues related to measuring big or small.