Sometime during the second trimester your doctor will start measuring your fundal height. Basically, it's how high your uterus is above your pubic bone. The measurement in centimeters is equal to the number of gestational weeks, plus or minus two weeks. So, for example, if you are 25 weeks along your fundal height should measure 23-27 centimeters.If the measurement is below that, there will be some concern that your baby is smaller than average, or that you less amniotic fluid than you should. If the measurement is above that, there will be concern that your baby is larger than average, or that you have more amniotic fluid than you should.This measurement isn't generally taken during the first trimester because the uterus does not grow enough to measure. During the first trimester an ultrasound will be done to judge the age of your baby, since at that point, the size of a fetus is standard for it's age. It's only as the baby gets bigger that the size will vary based a variety of different reasons.