How Big Does a Pregnant Belly Get in Different Trimesters?

Most pregnant women have a fundal height roughly equivalent to the number of weeks they are pregnant, reports Mayo Clinic. The fundal height, measured from the pelvic bone to the top of the uterus, is usually measured in centimeters. Since the fundal height can be 2 centimeters less or two more than the number of weeks of gestation, a woman who is 40 weeks pregnant should measure between 38 and 42 centimeters, according to Fit Pregnancy.

Most doctors begin to measure the pregnant belly around the 20-week mark, according to WebMD. Doctors use the fundal height measurement to determine the baby's growth rate and as an indicator for several health problems, says Fit Pregnancy. An abnormal fundal height can be caused by too much or too little amniotic fluid in the uterus, as well as by unusual fetal position close to the birth of the baby, reports Babble. However, most doctors do not rely on fundal height alone, instead using it as a guideline for whether it is necessary to perform an ultrasound. If the mother is overweight, carrying multiple babies, or is especially short or well-toned, her fundal height may not correspond with the growth of the baby or babies, says BabyCenter.