There are several variations of this kind of birth control, also known as the rhythm method or Natural Family Planning. Overall, they work when a woman notices her most fertile days of her menstrual cycle by the kind of cervical secretions in her vagina, as well as measuring basal body temperature (the body’s temperature upon waking) — these are collectively known as symptom-based methods. If a woman is trying to determine fertility based on the length of several menstrual cycles, it is called a calendar-based method. It is used for birth control when a woman actively avoids sex during her most fertile times of her menstrual cycle.

Related: Some Fertility Tracker Apps Are Useful, Study Says

FAM Option: Standard Days Method

This is only effective for women with two or more menstrual cycles that are between 26 and 32 days. (It doesn’t work when cycles are shorter or longer). Generally, the first fertile day is day 8 of a woman’s menstrual cycle, beginning with day 1 when the period first arrives. The last fertile day is day 19. Unprotected sex is avoided starting on day 8 and continuing through the end of day 19.

FAM Option: Two Day Method

This is when a woman checks her cervical secretions twice a day to determine her fertility. If there are no secretions, it is unlikely she is fertile and therefore is less likely to get pregnant, as sperm cannot move toward an egg easily and live up to five days in the vagina without such secretions. If she feels secretions, both on one day and the day after, she is likely in her most fertile period, and therefore more likely to get pregnant.

FAM Option: The Cervical Mucus Method

Cervical mucus has different consistencies throughout the menstrual cycle. Learning the differences can help you understand what's happening in your ovaries and in your cycle. There are several ways to check cervical mucus, and it can be done first thing in the morning. Wipe the vagina before urinating, observe mucus on the underwear upon waking, or reach into the vaginal opening with clean fingers to see and feel the mucus.

Decoding Your Body's Fertility Signals and Patterns

Recording the description of the mucus on a calendar or app will help you see patterns during the menstrual cycle. Menstrual blood may hide cervical mucus, so consider period days fertile ones. Once the period ends, there are typically dry days that may be considered less-fertile days depending on the length of your cycle. As an egg begins to get ready for ovulation, cervical mucus will appear, typically yellow, white, cloudy, and more or less sticky.

Slippery Cervical Mucus Means Fertility

Right before the egg is released (ovulation), mucus gets slippery so it can best help sperm move to meet an egg. This is the most fertile time and when it is best to avoid unprotected vaginal sex. After several days, cervical secretions get more cloudy and sticky, and then disappear. When there are no secretions, this time is considered infertile. This phase continues until the period begins again and the cycle starts over.

FAM Option: Cervical Mucus Plus Basal Body Temperature tracking

Another method often used along with cervical mucus tracking involves taking your temperature upon first waking up in the morning. The body temperature rises slightly when you ovulate, so tracking the rates for at least three months can help predict when you are most fertile (and when to avoid unprotected vaginal sex).

It is recommended to take your temperature at the same time in the morning every day, yet it is important to understand that sickness, stress, fatigue, and other factors can affect the body’s temperature. Consult a healthcare provider or counselor about tracking your observations and how to best interpret your chart for contraception purposes.

FAM Option: Combined or Sympto-Thermal FAM

You can call it the ultimate combo: Using two or three FAM approaches, such as cervical mucus observations, plus a calendar or basal body temperature charting to predict ovulation, the symptom-thermal fertility awareness method increases effectiveness and reduces failure rates compared with the use of one FAM method alone.

Learn More About Natural Family Planning and Fertility Awareness-Based Methods of Birth Control.

FAM Option: Lactational Amenorrhea Method (LAM) or Exclusive Breastfeeding

This method is when new mothers exclusively breastfeed their new babies for a period of up to about six months. Exclusive breastfeeding means the baby is nursed on demand (during the day and at night), does not use any bottles or pacifiers, is not eating any formula or other foods, and the mother hasn’t had her period return yet, not even spotting, since six weeks after delivery.

LAM efficacy rates are about 98 to 99 percent if the mother follows all the above conditions, but start to decline as soon as an infant eats more than breastmilk, nurses less than on demand, or the mother’s period returns. In these cases, using a second form of contraception, such as a condom, is a good idea to prevent an unplanned pregnancy.

The Bottom Line on Natural Family Planning and Fertility Awareness Methods of Contraception

Fertility awareness methods takes a lot of dedication, both in tracking changes over several months as well as avoiding vaginal sex (or using condoms) when most fertile.

How effective is it? FAM is not the most effective at pregnancy prevention: 76 to 88 percent.

It can cost $20 for supplies (a basal rate thermometer, pen and paper for tracking). ( 13 )

“I don’t think FAM is reliable,” says Bond. “There is pre-ejaculation that can contain sperm that you can become pregnant by. If you are not sure about having some kind of pill, or other hormonal contraception, you should consider using condoms, because they are easy and don’t require that much change in the body.”