When Does Pregnancy Occur?

Study Shows Few "Safe" Days During Menstrual Cycle

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According to researchers at the National Institute of Environmental Health Sciences, only about 30% of women have their fertile period between days 10 and 17 of their menstrual cycles.??

Researchers found that the potential for fertility exists on almost every day of a woman's menstrual cycle. However, most women in the study were between the ages of 25 and 35— the time of a woman's life when her menstrual cycles are at their most regular. The window of fertility was found to be more unpredictable for teenagers and women approaching menopause.

It's important to note that the study did not consider other aspects of fertility awareness that a woman can choose to use, such as charting her basal body temperature and cervical mucus (which increases the accuracy of the method).??

A Look at the Statistics

The researchers published their findings in the British Medical Journal. The study was conducted on 213 women during almost 700 menstrual cycles and concluded that even women with normal regular menstrual cycles should be advised that their fertile window can be unpredictable.??

Women who seek to use their cycles to avoid pregnancy might face poor odds, according to the findings. Data from the study suggests that there are "few days of the menstrual cycle during which some women are not potentially capable of becoming pregnant—including even the day on which they may expect their next menses to begin."??

According to researchers, "If the average healthy couple wants to get pregnant, they are just as well off to forget 'fertile windows' and simply engage in unprotected intercourse two or three times a week."??

Researchers showed that two percent of women started their fertile window by day four of their menstrual cycle, and 17 percent by day seven. More than 70 percent of women were in their fertile window before day 10 or after day 17.??

Women who regarded their menstrual cycles as "regular" had a one to six percent probability of being fertile even on the day their next period was expected to begin. This leaves few "safe" days for natural birth control methods such as the "rhythm method."

Having sexual intercourse on your fertile day does not guarantee you will become pregnant. Other factors—including the viability of the sperm and egg, the receptivity of the uterus, and other individual factors among couples—also influence whether pregnancy will result.

If you are sexually active and not ready to have children, these results highlight how imperative it is that you use another method of birth control.

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Article Sources
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  1. Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259-62.? doi:10.1136/bmj.321.7271.1259

  2. Planned Parenthood. Birth Control: Fertility Awareness Methods.?

  3. National Institute of Environmental Health Sciences. Study of normal women demonstrates: there are few "safe" days in menstrual cycle. November 2000.