A new study in the British Medical Journal confirms what women already know from experience’€“ that you can’€™t rely on the “rhythm” or “calendar” method for planning or preventing pregnancy.

The rhythm method is based on the understanding that a regular menstrual cycle lasts 28 days, that ovulation occurs 14 days before the next menstruation, and that a woman is therefore potentially fertile between days 10 and 17 of her menstrual cycle.

But while this logic holds in theory, it seldom works in practice, because a woman’€™s body does not work like a clock.

British researcher Allen J. Wilcox and colleagues studied a group of women who were trying to fall pregnant and found that only 30 percent of women were fertile on the days when, according to standard clinical guidelines, they should have been in their “fertile window”.

For the vast majority of women in Wilcox’€™s study (70%), the standard clinical guidelines predicting their fertile days were inaccurate.

In fact, says Wilcox, only 69 of the 213 women in the study had 28 day cycles and of these, only 10 percent ovulated exactly 14 days before their next menstruation.

The timing of the fertile window was even less predictable for the majority of women with irregular cycles.

But according to Prof Lynn Denny, a gynaecologist at Groote Schuur Hospital, this research merely confirms what many women, who have conceived while relying on the rhythm method, have already learnt the hard way.

Fertility experts already know that they cannot rely on the rhythm method for predicting ovulation. At the Groote Schuur infertility clinic, ovulation is artificially induced and then monitored before a timed insemination is performed, says Denny.

Proponents of natural family planning (NFP) methods argue that even if the rhythm method is obsolete, there are other NFP methods which take into account the natural variability of each woman’€™s cycle and are as reliable as artificial methods of contraception.

“The rhythm method came out of the ark and is no longer promoted by anybody who knows anything about NFP’€¦” says GP and natural family planning practitioner Dr Anne Williams in response to the BMJ article.

While the rhythm method simply attempts to predict ovulation mathematically, modern NFP practitioners also use other methods to establish exactly when ovulation is occurring. These involve assessing changes in the cervix, cervical mucus and basal body temperature.

But Dr Margaret Moss who heads the Contraceptive and Sexual Health Services at Groote Schuur argues that these additional methods rely on assessing physiological changes that are extremely difficult to interpret.

A rise in temperature may indicate the flu rather than ovulation, and fluctuations in cervical mucus occur for a variety of reasons, only one of which is ovulation, says Moss.

NFP also requires dedicated commitment from both sexual partners.

“You need to know your body inside out, keep daily records of all sorts of bodily functions and be able to abstain from sex for a fairly large proportion of each month,” says Moss.

“NFP has a high failure rate because it requires abstinence and we get carried away by our hormones. And for most South African women, with little control of their sexual activity, it’€™s a non-starter.”

But any method is better than no method at all, says Moss.

“If your beliefs or religious convictions don’€™t allow for other methods, then NFP is certainly better than nothing.”