I decided to peek around and see what I can come up with.
I did find an artcle related to this, and it is an
extremely rare complication that can arise with women who recently had a baby and had sex too soon afterwards (most doctors give a time frame as to when you're safe).
I think it's something like 6 weeks after childbirth that you are good to go. The article stressed (so I will too) that this is very,
very rare. Most women who have complications from having sex too early have to deal with infections.
Mummy, it sounds like you're in the clear if your youngest daughter is 13 months. You can also try positions that will let as little air into your vagina as possible (pretty much anything with your legs closer together) or even make sure your guy doesn't go all the way out during sex.