How Does Pregnancy Affect a Woman's Basal Body Temperature?

Basal body temperature charting is often used by women who are trying to conceive. In fact, many women who have not conceived after several months of trying are often encouraged to track their basal body temperature, or BBT, to find out more about their cycles. BBT is your body's temperature rest, and is best measured first thing in the morning when you get out of bed. Once you begin to move around, your core temperature increases measurements become less precise. Basal thermometers are designed specifically for this purpose, and often provide an extra decimal point of temperature reading for accuracy. When trying to conceive, it is helpful to track your daily BBTs on a chart over the course of a few months so that you or your doctor can look at the trends. This can help to pinpoint your most fertile days.

Your BBT is lowest early in your menstrual cycle, starting when your period begins. It may fluctuate during this time, but it usually stays within a few tenths of a degree of this baseline. When you ovulate, your temperature spikes half a degree or more, and remains high for the duration of the luteal phase. The luteal phase is the time between ovulation and the start of the next menstrual cycle. For many women, this phase lasts for 14 days, however yours may be shorter or longer. If a woman is not pregnant, her temperature will fall back to the baseline just before her next period begins, signaling the start of her next cycle. If a woman is pregnant, however, her BBT will remain high for the first trimester.

If your BBT has been high for a few days longer than your average luteal cycle, chances are very high that you are pregnant. In fact, most doctors will tell you that elevated BBT for longer than two weeks indicates pregnancy. However, this is only true if your normal luteal cycle is equal to or shorter than 14 days. If you are pregnant, you can expect your BBT to remain high for your first trimester, or about 12 weeks. After this, it will drop back down to your pre-ovulation baseline. Some women continue to chart BBT during their first trimester to ensure that it remains high. A drop in BBT before week 12 can indicate that there is a problem with the pregnancy.