Every couple is different, even in different generations of the family tree. Doing a doctor's consultation can give you a clue as to a particular peep's ability to have children, and the peeps will usually differ in their desire to have children. Even after working with Virtual Families for over 3 years on every platform we have offered the game on, my original advice still applies, if you want to have 6 children in every generation: stay involved in the family, try to make babies often, be persistent, and use baby boost when you have "money to burn."