First, she is very unlikely to get pregnant if she is using birth control correctly. the word "correctly" is important and means she isn't missing a day (I am assuming you mean the pill. There are lots of couples that use birth control pills as the only form of birth control. The pill is very reliable.
Second, it is highly unlikely to get pregnant from pre-cum. Pregnancy is not that easy, a lot of things have to happen for a couple to conceive. Generally this is done by ejaculation inside the vagina.
I think you are worrying way too much.