The reason BBT is a good way to track your cycle is because of what it indicates. After ovulation, your temp goes up because the progesterone that releases in your system. Around the time your period starts, it goes back down with the progesterone; sometimes that can be the day before your period actually starts, sometimes the progesterone lingers for a few days afterwards. Your temp stays up in the case of pregnancy because progesterone continues to be produced. Insufficient progesterone is a cause of miscarriage, which is why some women with a history of repeated miscarriages are prescribed progesterone supplements when TTC.
So in short: if your temp drops, you can fairly safely assume you're not pregnant. And if you did conceive and are able to get to the point of getting a positive HPT, then you need to talk to your doc about progesterone supplements to give yourself the best chance.
at 10:47 PM on Feb. 21, 2010