No, nothing special except (highlight for spoiler) the level 3 of Medicine (end of Spoiler)
And it's random!!

i've had preg. chiefs and sometimes use my male chief as my main man if he has the right hair colour. he is easier to find when i'm increasing pop. didn't know female preg chiefs could do the food. good to know as it can be very frustrating when your in that tough time.

Parenting skill, I believe, also helps get twins and trips.

You need L3 medicine and the rest is down to luck.

Luck must be the biggest factor. I have 3 villages all maxed out in the levels and have had only one set of triplets, even after multiple attempts at the "fertility" potions.