Fertility Awareness: Prevent or Promote Pregnancy, Naturally

This approach to fertility tracking is a big improvement on the rhythm method.

Posted Aug 17, 2020

Perhaps you’re old enough to remember the “calendar rhythm method" of birth control. Before the Pill became available in the late 1960s, the “rhythm method” was quite popular, especially among those who had religious objections to “artificial” contraceptives: condom and diaphragms, and later the Pill, IUDs, and others. Unfortunately, the rhythm method involved a distressingly high risk of unplanned pregnancies. 

That’s all in the increasingly distant past. Today, an updated, scientifically reliable version of the rhythm method, fertility awareness or natural family planning, teaches women (and couples) how to recognize the subtle but distinct signs of women’s fertile and infertile days each menstrual cycle. As a result, fertility awareness can be used to scientifically prevent—or encourage—pregnancy.

Why the Calendar Rhythm Method Is Unreliable        

Women are fertile for only part of each menstrual cycle. About midway between periods, they release eggs (ovulation). Fertilization can occur only during the week or so after ovulation, only around one week per month. Assuming women (or couples) can clearly distinguish between women’s fertile and infertile days, couples who hope to get pregnant can have lots of sex during fertile days, and couples who want to avoid pregnancy can abstain during fertile days or use nonhormonal contraceptives: condoms, spermicide, diaphragms, cervical caps, or non-intercourse lovemaking.          

To use calendar rhythm, women track the start of their menstrual periods for six to eight cycles. Each cycle begins on the first day of menstrual flow and ends on the first day of the next period. Ovulation occurs about midway between periods. Ideally, this tracking allows women to calculate when they’re most likely to ovulate and identify their fertile and infertile days each month.          

However, few women’s cycles are consistently regular, and many factors can change cycle-length and the timing of ovulation: travel, illness, drugs, fatigue, and stress—including worries about calendar rhythm’s reliability. As a result, the calendar rhythm method is better than nothing, but not particularly reliable.

Fertility Awareness Tracks Three Indicators

Fertility awareness uses the calendar plus two other elements—women’s basal body temperature and daily examinations of their cervical mucus. 

Basal body temperature (BBT) is temperature immediately upon waking and before women get out of bed. Typically, it’s a few degrees below 98.6. At ovulation, BBT spikes upward approximately one-half of one degree. This jump in body temperature can be identified easily and reliably using a special basal body thermometer that has an expanded scale in the temperature range around normal. If women’s cycles are irregular for any reason, the temperature spike is still a reliable indicator of ovulation. Basal body thermometers can be purchased online for $15 to $30. 

Another sign of ovulation involves changes in the consistency of cervical mucus. The cervix, the mouth of the uterus (womb), continually secretes mucus, which helps keep the vagina clean and receptive to sperm. But throughout the menstrual cycle, the texture of cervical mucus changes. During infertile days, it’s scant, white, and thick, which helps stop sperm in the vagina. When women are fertile, it’s copious, clear, and thinner, which encourages sperm to swim into the uterus to the fallopian tubes where fertilization occurs. Women can check their cervical mucus by inserting a finger and examining the consistency.

By combining the calendar, BBT, and cervical mucus observations, women can determine if they’re fertile or not—with confidence.                  

Fertility awareness works best in monogamous relationships when men understand the method and support it. Interested couples might take a class. Many family planning providers offer them. Or read a book, for example, Taking Charge of Your Fertility by Toni Weschler. Once trained, couples should invest six to eight months to learn the woman’s fertility pattern before attempting to use it for contraception.


  • Effectiveness: 80-99 percent. This means that if 100 couples use fertility awareness exclusively for one year, 1 to 20 pregnancies can be expected. Assuming the 100 couples make love once a week, that's 5,200 rolls in the hay. One to 20 pregnancies means contraceptive failures 0.02 to 0.4 percent of times. Reliability increases with training and commitment to using the method properly. 
  • Availability: Available to all couples, but requires training and months of observation before it can be used reliably.         
  • Side Effects: None. 


  • After purchasing the basal body thermometer and book or taking the class, fertility awareness is free.   
  • When used carefully, it’s highly effective.
  • No need for hormonal contraception.  
  • No need for any contraceptives for up to ten days each month.  
  • Sex can be spontaneous during infertile times. 
  • Can prevent or facilitate pregnancy.
  • Increases men’s familiarity with women’s menstrual cycles.        


  • When used improperly or carelessly, a significant risk of pregnancy.   
  • Takes six to eight months to master.
  • Other contraceptives are necessary during fertile times. 
  • No protection against STIs.
  • Vaginal infections and douching may change cervical mucus. (Women should not douche—see a previous post.)

Couples intent on preventing pregnancy should visit a family planning facility, discuss all the methods with a counselor, and choose the one (or more, e.g., diaphragm plus spermicide) that fits best into their sexual lives. 

Fertility awareness isn’t for everyone, but it just might work for you. My wife and I used it happily for 20 years.


Hatcher, R.A. et al. Contraceptive Technology (20th edition) Ardent Media, 2015.

Weschler, T. Taking Charge of Your Fertility. Morrow, 2015.