Both women and men have hormonal cycles which determine both when a woman can achieve pregnancy and when a man is most fertile. The average female cycle is twenty-eight days long, but the male cycle is variable. Women ovulate at about the fourteenth day of their cycle (though this varies), with the days leading up to ovulation being the woman's most fertile period. A woman can determine when she is most fertile by using a basal thermometer and charting cervical mucus characteristics. Women tend to dress more provocatively during ovulation to attract a mate.
Men can ejaculate and produce sperm at any time of the month, but their libido dips occasionally, which scientists guess is in relation to their internal cycle. Male testosterone levels surge when he detects odors of an ovulating woman, leading to an increased sex drive.
During the fourteenth week of fetal growth, the eggs (or ova) form in the ovaries of a female fetus, where they will remain until puberty. At puberty, the eggs will eventually start to mature one-by-one. At ovulation, the egg bursts from the ovary sometimes causing a small, sharp pain called mittelschmerz (German for "middle pain"). If the egg is not fertilized by the male’s sperm, the egg will break down within twenty-four hours into its components (mostly protein) and be reabsorbed by the body.