It was never shown in the anime or manga so it is unkown. well in episode what like 106-107?... (it may not be the exact episode but around there) when trunks appears and goku is just getting back...
There is no episode where it shows Bulma telling Vegeta she is pregnant. It just jumps ahead to baby Trunks, and so the details of those 3 years are left to one's imagination.
you can't haha jk some say it is because you dont really gain weight during the pregnancy or not much your stomach for some reason does not protrude as much, another could be you think you're having...