Your jersey is suppose to be light fitting around the waist so as to give you free movement during your delivery.
If your jersey is constantly being tucked in after your delivery, then its a poor fit to begin with. A problem that most every school and youth club has with uniforms.
However, many years ago there was a product that came out on the market that helped some guys with the uniform thing, and it was called "Hiddent Tailor". I'm not suggesting that you use it, because it can sometimes be just as much of a bother as the problem that your now having. But if you would like to do more research on your own, here is the web site that will explain the product more for you: