Fertile days 'hard to predict'

Predicting fertility is a hazardous business

Conventional wisdom which limits a woman's fertile period to a handful of days may be flawed, according to research.

Current guidance suggests that women can only get pregnant between days 10 and 17 of her menstrual cycle.

Variations not only occur between women but can happen for individual women from cycle to cycle

Anne Weyman, Family Planning Association

But a study published in the British Medical Journal suggests that only a third of women are fertile entirely during that period.

Scientists looked at the fertility of 213 healthy US women, testing daily urine samples for traces of hormones key to ovulation, the release of eggs prior to fertilisation.

They found that even women with "regular" cycles could not reliably predict when they were capable of conceiving, and when they were not.

In fact, a significant proportion of women actually began ovulating as much as three days before this "fertile window", and many remained fertile well after day 17.

Natural fertility control methods use a combination of checks to help a woman predict when it is safe not to use contraception.

These checks include testing for subtle changes in temperature every morning, and looking for other changes in the nature of cervical secretions which indicate when the fertile period is starting.

If taught to women by a professional, these methods have been shown to be 98% successful - that is, two out of 100 women using them will get pregnant within a year.

The research indicates that women should not simply regard the time between days 10 and 17 following their period as "safe".

Couples who are actively trying to have children should not simply have sex during this traditional "fertile period", but have sex two or three times a week across the whole month, say the researchers.

'Normal variations'

Anne Weyman, chief executive of the Family Planning Association said: "This is knowledge that has been used in natural family planning methods for many years.

"Variations not only occur between women but can happen for individual women from cycle to cycle. This is perfectly normal.

"However, it is this individual variation that can make it difficult to predict ovulation accurately solely by using calculations of cycle length."

She added: "For natural family planning to be as effective as possible it is recommended that other fertility indicators are also used, such as changes in the cervical mucus and body temperature.

"Natural family planning teachers are able to help people understand the reproductive process and teach them how to use this knowledge to plan or prevent a pregnancy."