The rhythm method was one of the first attempts at birth control through fertility planning. It is based on the concept that a woman cannot get pregnant during non-fertile days in her cycle, and allows women to either prevent or achieve a pregnancy. The rhythm method has been a form of birth control since at least 388 CE, where it was mentioned by St. Augustine in a letter.
This method is based on three ideas. The first is that the length of a woman's cycle is typically 28 days, give or take a couple days, and that a woman ovulates right around 14 days before her period begins. The second idea is that sperm can last three days in a woman's body. The third idea is that an egg must be fertilized within 24 hours of being released.
Using these three ideas, a calendar was developed, starting the first day of a woman's period, the day her cycle begins. A woman is considered fertile from the time her period ends until four or five days after she ovulates, usually around day 19. From day 20 until she starts her period again, she is considered infertile and cannot get pregnant.
There are a few advantages to the rhythm cycle. It is simple to use, and doesn't require the woman to remember to take a pill every day or use some other chemically-based birth control method. It is free, and has no side effects. It can be a useful tool to help a couple plan for conception, by allowing them to plan intercourse for the time when a woman is most fertile.
Unfortunately, the rhythm method is not an effective form of birth control. It is estimated that anywhere from 10% to 25% of couples using this method to prevent a pregnancy will conceive within a year. It doesn't take into account fluctuations in a woman's cycle, or that not all women ovulate on day 14 of the fertility cycle, and not all fertility cycles are about 28 days in length. The rhythm method also provides no protection against sexually transmitted diseases.
There are other fertility awareness methods of birth control that are far more accurate than this method. Charting basal body temperature, a woman's temperature when she first wakes up in the morning, monitoring changes in her cervical mucus, and checking daily for the position of her cervix can give a much clearer idea of when a woman is fertile. There are also specialized portable microscopes, available at some drugstores and online marketplaces, which allow a woman to check her saliva for ferning, a pattern in the dried saliva. This is a sign that ovulation is only a day or two away, allowing a woman to plan around her most fertile period.
