Modern rhythm contraception

Nathan and Kathy Sendan begin each day with a pen, paper and digital thermometer. The couple dutifully record Kathy's basal body temperature before they even think of drinking their morning coffee. Then they combine the temperature readings with other physiological data to track Kathy's fertility cycle and, in effect, to time sex.

Such is the routine for those who practise natural family planning, a method that shuns hormones, condoms and other artificial forms of contraception. It is the only form of contraception given the stamp of approval by the Catholic Church, but many proponents see a growing interest among non-Catholics as well.

Dr Joseph Stanford, assistant professor of family and preventive medicine, estimates that as many as 40% of those now practising this technique are non-Catholics. Natural family planning "offers an alternative where you don't have to mess up your physiology - you're more in tune with your body, and there are no side effects," Stanford says.

"It's not just a Catholic thing anymore", says Patrick Homan, spokesperson for US based Couple to Couple League that offers instruction on natural family planning. "Our numbers have been going up for the last five to six years."

Indeed, the Sendans are not Catholic, but they chose natural family planning because of dissatisfaction with the pill. "I liked the idea of not putting chemicals in my body", says Kathy Sendan.

She recalls being "grumpy all the time" during the three years she was taking oral contraceptives. She also had a more specific health concern: "I have epilepsy, and the [anti-seizuremedication could] have made the birth control pill less effective," she says.

Numbers remain small

The number of people choosing natural family planning still remains small. Advocates of natural family planning say their efforts are hampered by the stigma of the "old" calendar rhythm method, which relied on the expectation that ovulation occurs on day 14 of a 28-day cycle, and resulted in numerous "surprise" pregnancies.

In fact, menstrual cycles can vary from one woman to the next, and for many women, from one month to the next. Stress or illness, for instance, can disrupt even the most regular cycles. Such inherent variability was demonstrated in a study of 221 healthy women, published in the British Medical Journal in November 2000. Using daily urine tests to check for hormonal evidence of ovulation, researchers from the National Institute of Environmental Health Sciences (NIEHS) found that even though clinical guidelines assume the average woman is fertile between days 10 and 17 of her menstrual cycle, only 30% of the women studied had their window of fertility fall entirely within that time period. Even women with reportedly regular cycles had a 10% chance of being fertile "on any given day of their cycle between days six and 21," the researchers wrote.

"What was surprising to us is the fact that not only were fertile days coming early in the cycle, but late when a woman thinks she's on the end of her cycle," says Dr Allen J Wilcox, chief of epidemiology at the NIEHS and lead author of the study. "We're just putting numbers on something people had a sense of before."

The researchers also point out that most of the women in the study were between the ages of 25 and 35. Teenagers and women nearing the menopause tend to have even more unpredictable cycles.