How to Calculate Your Safe & Unsafe Periods

To avoid getting pregnant without the use of contraceptives, you can use the natural fertility awareness method to calculate your safe and unsafe times of the month. You will have to abstain from intercourse or use a barrier method of birth control during your unsafe periods.

Understanding the Cycle

The average menstrual cycle is approximately 28 days in length. The cycle begins with menstruation and a pre-ovulatory infertile period which lasts for approximately seven to nine days. The unsafe period then begins in the days prior to ovulation, which takes place approximately midway through your cycle, or cycle day 14. Because sperm can live within the vaginal environment for up to five days, unprotected intercourse generally must cease by cycle day nine. Once ovulation has taken place, a period of post-ovulatory infertility begins. Because it is difficult to know for certain when ovulation will take place, the unsafe window generally encompasses a broad range of days before and after ovulation to avoid an unplanned pregnancy.

Testing Your Body

While the 28-day cycle is a general menstrual cycle guideline, the length of a cycle varies from woman to woman, as does the day of ovulation. To accurately calculate your safe period, you must know your own fertile period. Note the first day of each menstrual cycle and record your temperature before getting out of bed each morning using a basal thermometer. Immediately after ovulation, there is an increase in temperature of approximately 0.4 to 1 degree Fahrenheit, according to the American Pregnancy Association. Stretchy, slippery cervical mucus is often present during your fertile period, too.

Alternatively, use an ovulation predictor kit to help pinpoint your unsafe and safe periods. These home-based urine tests register a surge in luteinizing hormone which takes place one to two days prior to ovulation.

Counting the Days

If you’re having difficulty pinpointing ovulation after a few months of monitoring your cycle, you can calculate your safe and unsafe periods using your cycle length. Subtract 18 days from your shortest cycle to mark the beginning of your unsafe period, advises the U.S. Department of Health and Human Services 3. Next, subtract 11 days from the longest cycle to mark the end of the unsafe period. For example, if your shortest cycle is 26 days in length, the beginning of your unsafe period begins on cycle day 8. If your longest cycle is 28 days in length, the end of your unsafe period begins on cycle day 17.

Calling in Backups

When the natural fertility awareness method is used correctly, it can be effective in preventing pregnancy as much as 90 percent of the time, according to the American Pregnancy Association 1. However, the rate of actual effectiveness averages around 75 percent due to inconsistencies in tracking and recording signs of fertility. While you’re figuring out your safe and unsafe periods, use a backup method of birth control to prevent pregnancy, and then continue using a backup barrier method during your unsafe period each month. Keep in mind that natural family planning does not protect against sexually transmitted infections.