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When Does Pregnancy Occur? Study Shows Few "Safe" Days During Menstrual Cycle

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Updated June 04, 2014

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Forget everything you have heard about when pregnancy occurs. According to researchers at the National Institute of Environmental Health Sciences, only about 30 percent of women actually have their fertile period between days 10 and 17 of their menstrual cycle. This adds validity to what many 'accidentally' pregnant women, including myself, have long suspected.

Researchers found that the potential for fertility exists on almost every day of a woman's menstrual cycle. Most women in the study were between the ages of 25 and 35--prime reproductive age and the age when menstrual cycles are most regular. The window of fertility was found to be even more unpredictable for teenagers and women approaching menopause.

Data on tests of 213 women during almost 700 menstrual cycles concluded that even women with normally regular menstrual cycles should be advised that their fertile window can be significantly unpredictable. The NIEHS' Allen J. Wilcox, M.D., Ph.D., statistician David Dunson, Ph.D., and epidemiologist Donna Day Baird, Ph.D., described the results of these tests of otherwise healthy North Carolina women in a recent report in the British Medical Journal.

Women who seek to use their cycles to avoid pregnancy may face poor odds, according to the new scientific report. 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 Dr. Wilcox, "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 2% of women started their fertile window by day four of their menstrual cycle, and 17% by day seven. Day one of the menstrual cycle is the day that menstruation begins. More than 70% of women were in their fertile window before day 10 or after day 17. Women who regarded their menstrual cycles as "regular" had a 1 to 6 percent probability of being fertile even on the day that their next period was expected to begin. This leaves few "safe" days for natural birth control methods such as the "rhythm method." Women who participated in the study were required to provide daily urine samples that were tested for estrogen and progesterone. A sudden change in these hormones marks the occurrence of ovulation. This test was used to identify ovulation days during 696 menstrual cycles.

Previous reports by Dr. Wilcox and colleagues had shown that the fertile window spans six days and ended on the day of ovulation; however the new report found that even women whose menstrual cycles are normally regular can experience sporadic ovulation. Late ovulation causes 4 to 6 percent of women to potentially be fertile during the fifth week of their cycles--more than 28 days after menstruation last started.

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

According to the NIEHS researchers, this report is the only modern test data that they are aware of on the range of the fertile window. During World War I, a German physician reported 25 pregnancies that were produced by single acts of sexual intercourse that occurred between menstrual cycle days 2 and 30 while soldiers were on leave--today, 84 years later this study suggests that the doctor's report was entirely plausible.

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