Basal body temperature, or BBT, is the lowest temperature a person's body will be throughout the day. Typically, BBT is taken first thing in the morning, before a woman becomes active and her temperature increases. Women who wish to become pregnant can determine when the best time to attempt pregnancy is by shifts in their body temperature.
The best way to get an accurate basal body temperature reading is to take the temperature before getting out of bed in the morning. Ideally, a woman will take her temperature at the same time each morning after getting a decent amount of sleep. It doesn't matter whether she takes her temperature orally or rectally, as long as she is consistent in her method. She shouldn't take her temperature orally one week and rectally another, for example. As an accurate temperature reading is only possible if a woman hasn't been active yet, she should plan ahead and keep a BBT chart and thermometer by the bed so that she doesn't have to get up and look for it in the morning.
Typically, a woman's basal body temperature is around 97.2 degrees Fahrenheit (36 degrees Celsius). At the start of ovulation, it will rise 0.4 degrees. Usually, a woman's basal body temperature increases by up to 1 degree Fahrenheit a few days after she ovulates. Some women may notice a drop in temperature just before ovulation begins, but this doesn't happen to every woman. If she tracks her BBT for a few months before attempting to become pregnant, she can begin to recognize a pattern in her temperature and use the pattern to determine when she will ovulate.
In most cases, the BBT stays high for up to 12 days after a woman ovulates. If her temperature remains elevated, it can be a sign that a woman is pregnant, especially if her temperature remains high for more than 18 days. Some women will notice an additional increase in temperature. This increase doesn't necessarily indicate pregnancy, though. In some cases, a woman's basal body temperature can move through three phases instead of the usual two.
If a woman's BBT doesn't remain elevated for 12 days after ovulation, it is usually a sign that a woman has luteal phase defect, which can cause infertility. The luteal phase is the timing between ovulation and the start of a new cycle. A luteal phase defect usually indicates that a woman isn't producing enough progesterone, which can interfere with the production of the endometrium lining, potentially making conception difficult.