I also agree, that if you mean she's on birth control it's a very bad thing to try and counteract that. She obviously is doing it for some reason, and is something you both would have to discuss. If she became pregnant and didn't want to, she could resent the baby or have some form of abortion so she didn't have to carry it. You both need to talk about this and figure out what you want to do.
Do you mean birth control???
Do you mean she is secretly taking the pill? To prevent pregnancy? If so I don't think there would be anything you could to prevent this. Plus that would be like 'drugging' your wife up so she t can get pregnant which would be extremely wrong. I suggest you talk to your wife about this and see why she is taking these. Seems like she isn't ready for a baby whereas you are, and this issue will only solved by talking to each and not going behind each others back.