I try to have a lot of my female of age villagers get preggers around the same time. That way, I hopefully won't have to worry about not having any of breeding age at any time. And once a baby is two years old, I sometimes have that mother breed again. I keep trying to get twins or trips, but so far, I've only gotten like 50 some odd single births. Do you have to have level 3 medical to get multiple births?