Does your stomach grow on second life when your pregnant

It can.
The appearance of your avatar can change as often as you want in Second Life. Lots of people who want to roleplay being pregnant change their appearance to look pregnant. Your avatar appearance is basically defined by two things: Your body "shape" and your "skin". In this case, as you progress in your pretend pregnancy, you could change your "shape" to make your body bigger in all the right places, and keep changing it as you grow. You could also change your "skin" to give the visual appearance of change in the color and look of your skin.