It has nothing to do with your stomach looking pregnant. What it means, is taking a poop is similar to giving birth to a baby. Different part, but similar symptoms.
You have no idea how long you will be in the bathroom (labor). It may be very painful or maybe not (delivery).
You have no idea about its size or what it looks like, until its born. A foodbaby is a turd or poop. you can say you look pregnant, but the belly sticking out is not called a food baby.
To go & take a poop, you can say I'm gonna give birth to a foodbaby.