Four hormones control the menstrual cycle. The delicate balance of these hormones in the blood regulates the sequence of events leading to ovulation and, if the egg is not fertilized, menstruation. The next problem is to pinpoint the time of ovulation. It is possible to do this with a reasonable degree of accuracy, given sufficient motivation and intelligence. In summary, fertilization can occur only around the time of ovulation.
Ovulation occurs in response to changes in the hormonal balance regulating the menstrual cycle.
The hormonal balance itself is easily affected by external factors. There is no way of predicting the exact time, and the best estimates are often in error. Rhythm methods are not effective. The rhythm methods of birth control are effective in certain cases. The calendar rhythm method is the least effective of the three rhythm methods.