I would say it just depends, but it is much more common when my family in incomplete. So, for example, if I have a family that consists of a couple and a toddler, but I want them to have a final baby, they would naturally woohoo more than if their family was already complete. I have risky woohoo, so after the family is complete I limit my sims to one non-birth control woohoo a week. Even then I get really scared of them having another child :D So yes, it depends on many things, whether it’s a family, couple, elderly grandparents, (personalities too),...
