First, are you certain of how many weeks along you are? At 20 weeks gestation (about 5 months), we expect your uterine fundus to be as high as your belly button.
Some women carry their pregnancies in a very "posterior" fashion, so may not "show" as much as other pregnant women. However, your physician should be able to feel the fundal height
at this point; check with your doctor.
Answer assisted by David Escobar, Medical Student