When your belly falls over your belt line.

When a really skinny person (usually male) eats too much food, their stomach visibly protrudes outward forming a belly that looks quite like a preggers woman.

Really they are just a fatty.

Effect is enhanced by tight t-shirts.

Foodbaby is actually a term in the medical field.

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.

1. A food baby is what happens after all the food you eat is digested and comes out of your butthole, this "food baby" then continues feeding others, in many different ways.

when someone has a larger stomach which resembles being pregnant

When you poop, a food baby comes out.

A colloquialism used to describe waste product from an animal's digestive tract expelled through the anus (or cloaca) during defecation. See also Feces.