Besides reading the baby books, both parents ... what else is there?
What about all people in the household reading the baby books?
What about interactions with the belly? How many of which interaction, and who by effects what? Rub, talk to ... there's a third one isn't there?
And eating certain things increases the chances of a boy or girl. In Sims 2 cheesecake guaranteed twins, but is there a food for twins in Sims 3 or just the happiness reward - fertility?
Anything else?
DivinylsFan Are you specifically asking about actions that increase the chance of multiple births, or anything that changes the outcome?
For gender, an apple offsets the gender chance by -0.1 and a watermelon offsets the chance by +0.1. A total value of -0.5 guarantees a boy; +0.5 guarantees a girl. Cooked foods don't affect gender unless you have a mod that adds that feature back.
For multiples, the base chance of having twins is 2.5% and triplets 0.1%. (Quadruplets are impossible without help from a mod.) Having the pregnant sim listen to kids' music or watch the kids' TV channel increases the odds, with a maximum multiplier of 30, increasing at a rate of 2 per hour. So that's a maximum chance of 75% for twins and 3% for triplets. The Fertility lifetime reward offers a flat permanent bonus, in addition to increasing the overall odds of conception (75% by default); the massage table from the Store offers a bonus for the duration of the fertility moodlet it grants.
Having the baby at the hospital gives a +20 bonus to the outcome of the pregnancy, as in, whether it goes well enough for the player to be able to choose traits, or barring that, whether the game-chosen traits are positive. I'm not sure about the total positive points (including what the sim gets from having a high mood) required to get the best outcome. Getting medical advice at the hospital is supposed to help too, but I don't see an associated value in the XML file.
The master tuning file lists a +15 bonus from the pregnant sim reading a pregnancy book, and the same for the father/other parent. But it's not clear whether this applies to the chance of twins or the outcome of the pregnancy. It would be easy to test though; let me know whether you're interested.
