How soon after having a baby?

Okay I am a bit of a hypochondriac. I am breastfeeding and so I have not had a period which I know to be normal. We are using birth control methods but I still want to take a pregnancy test every now and then just so if a little miracle happens we aren't half way through the pregnancy before finding out. My son is 11 weeks old so if I take a test it won't be affected by the hcg from my son will it?