The journey from the exit out of the male body to the outer gates of the female egg is like a dance marathon for sperm. When the starting gun sounds, 150 million swimmers head for the finish line, but only 15 percent of them on a good day are fit enough to make it past the uterus to the Fallopian tubes where the prized egg awaits [source: Angier]. By that time, only 1,000 or so have survived, and from there, just the strongest dozen reach the final destination [source: Freudenrich and Edmonds]. Not to mention the biggest wild card of all in the reproductive landscape: whether the ovaries have even released an egg into a Fallopian tube.
Before the late 1920s, doctors had modeled women's fertility on the mammalian estrous cycle, in which females go into heat, since the role of hormones was unknown at the time. In some species, if the cyclical window of fertility doesn't result in pregnancy, endometrial lining will shed, similar to having a period. By that logic, scientists presumed that menstruation and ovulation were likewise co-dependent, or that ovulation merely happened spontaneously [source: PBS]. The research team of Edgar Allen and Edward A. Doisy isolated and identified the roles of the hormones progesterone and estrogen in 1927 and 1928, respectively, by examining the ovarian tissue in female rats [source: Phillips]. By unearthing those key hormones, scientists were able to map out the menstrual cycle as well as ovulation.