In reality it is impossible to impregnate anyone or give birth through the anus. Mentioning or discussing the concept of a butt baby is a pursuit into often intentionally juvenile humour.
Rick: "Hey, us butt babies are contributing members of society! We are all equal, whether born from the hoo-ha or born from the ha-hoo."
Gestation occurs in the lower intestine and the baby is born through the anal sphincter which dilates (to up to a foot) in diameter for this purpose.
2)An insult directed at people who enjoy rectums and their function.
2)"Thomas is such a Butt Baby, look at the way he acts around rectums."