A fertility calendar is a way for women to chart their monthly cycles. It can be used as both a method of birth control or as a fertility method. Learning how to use and read a fertility calendar is easy. By inputting a few numbers, a fairly accurate calendar of a woman's fertile days can be created.
Also called the rhythm method, fertility calendars are designed around a woman's personal monthly schedule. It takes into account that each woman's cycle is unique. For example, the length of a woman's cycle averages twenty-eight days. However, it can be as short as ten days, or as long as sixty days. Some cycle lengths vary from month to month.
Fertility calendars help to track and measure the length of a cycle. For a woman trying to conceive, it is important to know when she is ovulating. This is just as important for a woman who is trying not to get pregnant. By keeping track of the first day of her period for a couple months, a woman should be able see a pattern to her monthly cycle.

By using the length of her cycles, and the first day of her most recent cycle, a fertility calendar can tell a woman when she will be most fertile. Some charts can even predict when the best time to achieve a boy or girl will be. Y-sperm (male) move faster than X-sperm (female). However, X-sperm live longer. So having intercourse a few days early will be more likely to result in a female child, because the Y-sperm will have died off by the time the egg is released. Waiting a few days for intercourse, until a day before ovulation, will give the faster Y-sperm a better chance of reaching the egg first. Abstaining from sex during the fertile time will prevent a woman from becoming pregnant.

A fertility calendar can also tell a woman when her period is next due, as well as when to test for a pregnancy. They can even tell when the baby will be due, based on the estimated date of conception. Fertility calendars have helped millions of women conceive, but they are not perfect. They work on the assumption that women ovulate halfway between their cycles, which is not always true.

A more accurate picture of when ovulation will happen is gathered by taking the idea of a fertility calendar one step further. By charting her cervical position, vaginal secretions, and temperature, ovulation can be more accurately predicted. Because the female body changes in definite ways through the different stages of a woman's cycle, it is possible to track a woman's fertility. It is called basal body temperature (BBT) charting, and is recommended by a lot of fertility doctors for couples having a difficult time conceiving. With a fertility calendar, women are able to plan for a pregnancy. Women who know when their fertility window is have a better chance of conceiving than a woman who is just guessing on the best times to have intercourse.