How does the rhythm method of birth control work?

Also called fertility awareness, this method of birth control consists of avoiding sex during a woman’s fertile period: the time when the egg leaves the ovary and enters the fallopian tube. You can determine your fertile period by determining your ovulation on a calendar, by using a thermometer to take your temperature every morning (a small rise in temperature occurs after an egg is released), or by noting when vaginal mucus is clear and thin. This method does not protect against sexually transmitted diseases. Although this method is preferable for women whose personal beliefs prohibit other methods of birth control, it is unreliable for a woman who has irregular periods, a woman who has frequent pelvic infections, or a woman who cannot interpret her fertility signs correctly.

The rhythm method of birth control involves mapping your menstrual cycle to find your fertile days and then avoiding sexual activity during those days and the two days prior. You have to be half mathematician and half magician to make this work for any length of time. 

Your ovulation day is, on average, 14 days before your period starts. From there, you have to back up two days to offset the life cycle of the sperm. The problem here is obvious. Unless you have solidly predictable cycles naturally, you have no idea when your next period will start so you can count back two weeks. You also cannot predict if and when you may have a spontaneous ovulation. It can happen 'in the heat of the moment'. Then all your counting is for naught. Finally you have to be strong enough to say no to sexual activity at a time when your hormones are pushing you in the opposite direction.

In summary, unless you have no other options, rhythm is the least reliable method you could pick. It relies on unreliable information and weak-willed people to make it work - not a very successful combination.