First remember that babies come in all sizes, as do bellies. Your doctor or midwife is monitoring your baby's size in relation to your due date and the size of your pregnant belly through a series of screenings, including measuring your abdomen at every visit once you reach 15-20 weeks. This measurement tells them how much your belly is growing. Really strong abdominal muscles can prevent your abdomen from "hanging out" and make you look smaller, despite having a perfectly appropriately sized baby. In rare cases, you may have less amniotic fluid, called oligohydramnios. If you had this your doctor or midwife would tell you at your prenatal appointment and you would not measure correctly.