Women's fertile window very unpredictable -- study

LONDON, England (Reuters) -- Women who think they can accurately predict the days of the month when they are most fertile should think again.

Researchers at the National Institute of Environmental Health Sciences in Durham, North Carolina, said Friday that a woman's fertile window -- the six days each month when she is most likely to conceive -- is extremely difficult to predict.

Even women with normal, regular menstrual cycles can have erratic fertile windows that don't occur when they might expect.

"There is a peak probability for women in the mid-cycle but the probability doesn't end there, it extends into the very early and the very late part of the menstrual cycle," Dr. Allen Wilcox, an epidemiologist at the institute, said in a telephone interview.

He and his colleagues analyzed the monthly cycles of 213 women who were trying to conceive. Medical guidelines suggest that women are most fertile and likely to ovulate half-way through their cycle, usually days 10-17.

It is generally the best time to have sex if a woman wants to get pregnant and the worst if she doesn't.

But only 30 percent of the women who took part in the study had their fertile window entirely within the six days of their mid-cycle.

"We were surprised to see how variable it (the fertile window) was," said Wilcox, whose research was published in The British Medical Journal.

"Even women who said their cycles were regular and every 28 days had a lot of variability."

He added that careful studies have not been done on ovulation in a large group of women to pin down how much variability there is.

Wilcox advised women who are trying to avoid pregnancy not to use the calendar method because it is very unreliable, particularly for those women with irregular cycles.

"If they are not inclined to use birth control they should be very, very careful about using a detailed method of monitoring their own biology instead of just relying on the calendar," he said.

But if a woman is trying to conceive and she doesn't have any reason to be concerned about her fertility, she should ignore the study and let nature take it course.

In a separate study in the journal, doctors at the Latin American Center for Perinatology and Human Development in Uruguay said women with very short or long gaps between pregnancies have a higher risk of complications.

In a study of 400,000 women in Latin America and the Caribbean, gaps of less than six months were associated with higher risk of maternal death, anemia and infection and bleeding in late pregnancy.

Women with gaps of 59 months or longer between pregnancies were more likely to develop pre-eclampsia and eclampsia, potentially fatal complications for both the mother and child.

Copyright 2000 Reuters. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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National Institute of Environmental Health Sciences
American College of Obstetricians and Gynecologists
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