If you don't fertilize the egg during ovulation, the surrounding follicle becomes a corpus luteum. This structure increases progesterone levels, starting menstruation. Progesterone release stops when the corpus luteum becomes a corpus albicans; this tissue is what comes out of you during your period.
A side effect of progesterone is increased body temperature, which makes you burn calories. A 1995 study by the University of British Columbia found that on average women will eat an additional 265 calories per day to compensate for this effect, although a few women in their trial increased their food consumption to as much as 500 calories per day.
This change in temperature is also used by women trying to conceive. By measuring Basal body temperature, the body's lowest temperature reached during sleep*, a woman can find out when menstruation first occurs. While physical signs of menstruation may take a few days to develop, the temperature change from increased progesterone can show up as soon as the day after ovulation ends. By tracking these changes, women can estimate how long they have to conceive after ovulation has started. During menstruation, body temperature will go up 0.4F (0.2C.)
That's right: It takes an average of 265 extra calories to warm your body less than half a degree.
*It's hard to measure your own body temperature when sleeping, so it's done as soon as you wake up.
Posted 2195 day ago