Fertility awareness (also called natural family planning or periodic abstinence) involves monitoring the changes your body goes through during a menstrual cycle. This information can help you identify when you ovulate and can be used to time sexual intercourse either to avoid getting pregnant or to increase your chances of becoming pregnant.

For fertility awareness to be an effective method of contraception, you must abstain from sex or use a barrier method of contraception (such as a diaphragm or condom) for 8 to 16 days of every menstrual cycle. This method of birth control requires organization, close observation of your body changes, and cooperation from you and your partner.

The menstrual cycle

The menstrual cycle is the regular sequence of changes caused by hormone levels that affect the uterus and ovaries. A complete menstrual cycle lasts 24 to 35 days in most women.

• Day 1 is the first day of menstrual bleeding (menstruation or menses). Menstrual bleeding occurs when the blood-filled lining of the uterus (endometrium) is shed.
• On days 7 through 14 (up to day 21 in a 35-day menstrual cycle), the lining of the uterus develops and thickens to prepare for pregnancy. One or more eggs begin to develop in an ovary (inside a follicle). This is called the follicular phase.
• About 12 to 16 days before the next menstrual period begins, an egg is released from an ovary (ovulation). The lining of the uterus (endometrium) prepares to allow a fertilized egg to attach (implant) to the endometrium. This is called the luteal phase. If the egg is not fertilized, the endometrium (and the egg) are shed during the next menstrual period. 

A woman is usually able to get pregnant for about 5 days each month, when ovulation occurs. On average, ovulation occurs 12 to 16 days before the menstrual period begins. Therefore, ovulation would occur on about day 10 of a 24-day menstrual cycle, day 14 of a 28-day cycle, or day 21 of a 35-day cycle. The key to becoming pregnant or to avoiding pregnancy is determining when ovulation will occur. Because sperm can live for 2 to 7 days in the a woman's reproductive tract, it is possible to become pregnant if intercourse occurs several days before ovulation.

Detecting ovulation

There are several basic methods for determining the time of ovulation.

• Calendar (rhythm) method. For the calendar method, you estimate ovulation based on a record of your previous menstrual cycles. From the record, you predict which days of the month you are most fertile. Your fertile days start 5 days before ovulation. This method assumes that your cycle is regular and that you will ovulate on a certain day of the month. However, very few women actually have regular 28-day cycles, and even those who do can have irregular periods from time to time. Also, a woman does not always ovulate right in the middle of her cycle. The best estimate is that ovulation occurs between 9 and 17 days before the next period. Therefore, use of the calendar method alone, without checking for the signs of ovulation, is not recommended as a method of birth control.
• Basal body temperature (BBT) method. Basal body temperature (BBT) is the lowest body temperature a healthy person achieves during a day. Because of the natural changes that occur in a woman's hormone levels during her menstrual cycle, her BBT falls 1 to 2 days before ovulation and rises 1 to 2 days after ovulation. By carefully measuring and charting your BBT every morning before you get out of bed, you may be able to estimate the time of ovulation.
• Cervical mucus method (Billings method). The amount, texture, and appearance of mucus produced by your cervix changes during your menstrual cycle. By observing, feeling, and recording the nature of the mucus over several cycles, you can predict when ovulation occurs.
  • Right after the menstrual period, cervical mucus is sparse, thick, cloudy, and slightly sticky.
  • Just before and during ovulation, the amount of mucus increases and it becomes elastic and stretchy, slippery, thin, and clear.
  • Hormone monitoring. Home ovulation kits are available without a prescription to help you identify the most fertile days during your menstrual cycle. These tests measure the presence of luteinizing hormones in your urine with a dipstick or test strip; then, on a small computerized monitor, they provide information about your fertility level. Although expensive, these monitors tend to predict ovulation better than the BBT or cervical mucus methods. To avoid pregnancy, it is important to estimate your most fertile days, even when using a home ovulation kit.
  • Combined (symptothermal) method. This approach combines several methods of monitoring your body's changes during your menstrual cycle, such as measuring your basal body temperature, observing the changes in your cervical mucus, hormone monitoring, and watching for signs of ovulation (such as breast tenderness, abdominal pain, and mood changes). Predicting the timing of your ovulation is most successful when using a combined method.

  Fertility Awareness
  Results

  Generally, your most fertile days begin 5 days before ovulation and end on the day of ovulation. Pregnancy can occasionally occur after ovulation, but it is less likely than in the days before ovulation.

  Calendar (rhythm) method

  If your menstrual cycle is 28 days long, you are most likely to ovulate about 14 to 15 days after menstrual bleeding begins. However, this is not very precise, especially if you have an unusually short, long, or irregular menstrual cycle.

  The calendar method of birth control is not very precise, especially for women who have unusually short, long, or irregular menstrual cycles. For this reason, use of the calendar method alone, without using other methods of detecting ovulation, is not recommended as a method of birth control.

  Basal body temperature (BBT) method

  Your basal body temperature usually drops about 0.4°F (0.22°C) below your normal temperature 1 to 2 days before ovulation. It then increases the same amount or more above your baseline temperature 1 to 2 days after ovulation and remains there until just before your menstrual period begins. Since the rise in BBT does not occur until after ovulation, it is possible to become pregnant if you have intercourse just before or during ovulation.

  Unfortunately, many women do not show this classic temperature pattern, making it difficult to predict ovulation accurately.

  When the basal body temperature (BBT) method is used as the only method of birth control, it has been shown to be 80% to 93% effective (meaning about 70 to 200 women out of 1,000 who use this method for a year will become pregnant). If vaginal intercourse occurs only after the temperature pattern indicates that the fertile days are over, the effectiveness increases.

  Cervical mucus method (Billings method)

  After menstruation, cervical mucus tends to be very scant, cloudy, whitish to yellow, and slightly sticky. Just before ovulation the amount of mucus increases, becomes clear and slippery, and stretches more than 1 in. (2.54 cm).

  When cervical mucus observation is used as the sole means of birth control, it is usually about 80% effective (meaning that about 200 out of 1,000 women who use this method for a year will become pregnant). However, if sexual intercourse is avoided for 3 days after the cervical mucus indicates ovulation, the effectiveness increases.

  Hormone monitoring

  Computerized home ovulation tests are available to measure the amount of luteinizing hormone (LH) in the urine and to display the results on a small computerized monitor.

  When used as a means to predict fertility, hormone monitoring can be about 94% effective (meaning that about 60 out of 1,000 women who used this method for a year will become pregnant).

  Combined (symptothermal) method

  Many women experience symptoms such as slight vaginal bleeding (spotting), breast tenderness, swelling of the vaginal lips (vulva), abdominal bloating, sharp twinges of lower abdominal pain on one side, and increased sexual desire around the time of ovulation.

  If the combined (symptothermal) method is used to determine ovulation, the effectiveness of birth control is nearly 99%, provided no unprotected intercourse occurs from several days before ovulation until several days after ovulation. Effectiveness is lower if sexual intercourse occurs both before and after ovulation. However, this method is difficult to use for a woman who has had a baby within the past 6 months or who has an irregular menstrual cycle.