The rhythm, or calendar, method of birth control, assumes a woman is fertile between day 10 and day 17 of her cycle. According to the theory, unprotected sex at any other time in the woman’s cycle won’t result in pregnancy. Recent research, however, shows that this method works even less reliably than previously believed.

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Researchers at the U.S. National Institute of Environmental Health Sciences analyzed 700 menstrual cycles in 213 healthy women. Rather than relying on the women’s own accounts of their last period, the scientists measured the levels of estrogen and progesterone in urine. KSL-TV in Salt Lake City reports that fewer than one-third of women had periods regular enough for the rhythm method to work.

This news also affects couples who are trying to conceive. Many of them use the rhtythm method to time intercourse to coincide with peak fertility. However, calculating the day of ovulation can be tricky. “Many women do not have the textbook example of a 28-day cycle,” says Dr Laura Castleman, an obstetrician-gynaecologist in Troy, Michigan. “Some women have a 35-day cycle or a 21-day cycle. It’s different for every woman.”

If couples are wanting to conceive, but fail, they should seek help if nothing has happened within a year.

Read more:
The injectable contraceptive debate
Any questions? Ask our sexologist

 

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