An inexpensive way of detecting ovulation through the effect progesterone has on the hypothalamus, increasing the body's temperature a few tenths of a degree. Best recorded using a BBT thermometer (with a smaller scale than thermometers used to record fevers or high temperatures) first thing in the morning before rising, and preferably in the vagina for accuracy. Day 1 of the chart is the first morning when there is menstruation (a period). Commonly there's a dip in the temperature just before the sustained rise that indicates that ovulation has occurred (a "biphasic chart"). The chart typically records the days you are menstruating, when you have sex and when you're aware of mucus and ovulation pain. Best used to document: (1) the presence and length of the luteal phase, especially if clomiphene is being used for ovulation induction; and (2) the timing of symptoms such as premenstrual spotting. Not as good for predicting ovulation as LH-testing in urine.