Basal Body Temperature.

**Please note this is Copied from Wiki & possibly other sites & reworded**

Basal body temperature is the lowest temperature attained by the body during rest (usually during sleep). It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature. In women, ovulation causes an increase of one-half to one degree Fahrenheit (one-quarter to one-half degree Celsius) in basal body temperature (BBT); monitoring of BBTs is one way of estimating the day of ovulation. The tendency of a woman to have lower temperatures before ovulation, and higher temperatures afterwards. Charting of this pattern may be used as a component of fertility awareness.

While trying to conceive

Regular menstrual cycles are often taken as evidence that a woman is ovulating normally, and irregular cycles is evidence she is not. However, many women with irregular cycles do ovulate normally, and some with regular cycles are actually anovulatory or have a luteal phase defect.

Records of basal body temperatures can be used to accurately determine if a woman is ovulating, and if the length of the Post-Ovulatory (Luteal) phase of her menstrual cycle is sufficient to sustain a pregnancy. Some fertility computers and software can help a woman to determine these factors. (aka Fertility Friend) Pregnancy Tests are not accurate until 2-3 weeks (14-21 days) after ovulation. Knowing an estimated date of ovulation can prevent a woman from getting false negative results due to testing too early. Also, 18 consecutive days of elevated temperatures means a woman is almost certainly pregnant.
Tracking basal body temperatures is a more accurate method of estimating gestational age than tracking menstrual periods