During anal sex, the man ejacualtes into the anus of the male or female receiver. When the receiver later uses the toilet, what comes out of the anus, a mixture of ejaculate and feces is a poobaby.
Chris and Sean, sitting in a tree,
K-I-S-S-I-N-G,
first comes love, then comes marriage,
then comes the poo baby in the toilet carriage.
When you eat too much the night/day before - and your bloated stomach's contents have digested enough - and your 'leftovers' are ready to be "dropped off at the pool".
Nat - "I ate too much last night! Poo Baby is ready to launch!"
Kim - "Same here, I'll come with."