Because a woman's breasts grow during pregnancy, I reckon you could add 'breast expansion' as a theme... Especially if all of the characters were to conveniently keep their bigger boobs after childbirth (which, of course, can definitely happen in real life)... So, then you just develop (pun not intended) that "more impregnation" theme and you'll be laughing! Heck, the big boobs would/could run in the family so it means that the female offspring would be top-heavy at a young age--you could even depict that, no need for any sex acts... ^^