I also take both.

Like others, I'd HIGHLY recommend talking to both your ob and your psych doc about this. But I just thought I'd give you my personal experience.

I was told that during the three weeks of the "normal" hormone pill, the Lamictal's effectiveness is reduced by as much as half. In my experience, I don't feel that Lamictal does very much to stabilize my moods and this is most likely why. It is also ironic that during my "off week" is the most likely time that my moods would be out of control just from the natural hormones. But I've noticed that this is the time of the month when I feel the absolute best.

My psych doc hinted that at the next visit, if I wasn't seeing an improvement overall I'd be switched to Lithium.

I continue to take birth control because the other psych med I'm on (Paxil) would have very bad consequences on an unborn child. Primarily for this reason, my doctor did not want me to discontinue my birth control. She also told me that while birth control affects Lamictal's effectiveness, it does not work the other way around. Your birth control does not lose any effectiveness from the Lamictal.