For years the birth control methods collectively known as periodic abstinence have been jokingly referred to as "Vatican roulette," a nod to the fact that these techniques are both Vatican approved and quite likely to end in pregnancy. (The World Health Organization reports that on average, women practicing periodic abstinence for a year have a one in four chance of becoming pregnant.)

A new German study, however, has found that, when practiced correctly, a method of periodic abstinence known as the sympto-thermal method (STM) leads to an unintended pregnancy rate of only 0.6 percent annually. This rate is comparable with that of unintended pregnancies in women who use birth control pills, the most popular method of contraception in the U.S.

For the sympto-thermal method to work, women must keep track of three things: their core body temperature, the fertile days of their cycle as measured by a calendar, and their cervical secretions. Using this information, women are able to abstain from sex during their fertile period, which includes several days prior to and after the day of ovulation. According to lead study author Petra Frank-Herrmann, a fertility researcher at the University of Heidelberg in Germany, STM is more effective than the other so-called periodic abstinence methods because it uses more than one type of information to predict the dates of a woman's fertile period.

As effective as STM can be, experts say it is not right for everyone. Whereas the method is cheap (read: free) and appeals to women who want a natural birth control method, it requires a strong commitment on the part of both partners.

"You really can't extrapolate from this paper," says David Grimes, an obstetrician-gynecologist and vice president of biomedical affairs at the nonprofit public health organization Family Health International. "Naive readers see these results, and they think [STM] is the greatest thing since laptop computers. The researchers on this paper went back and cherry-picked this data from an ongoing study from the past 20 years. They chose the users who were the best users for this method."

Hilda Hutcherson, an ob-gyn and co-director of the New York Center for Women's Sexual Health at New York-Presbyterian Hospital/Columbia University Medical Center, found that her patients often stop using periodic abstinence methods after only a few months. "It's difficult to abstain from sex for two out of four weeks," she says. "That means half the month you can't have sex. That's very difficult for young couples." (It's worth noting that STM actually requires only 7-10 days of abstinence, but related methods of birth control still practiced in the U.S. do require up to two weeks of abstinence every month.)

Grimes of Family Health International believes that studies of periodic abstinence are often motivated in part by religious beliefs. "Many of the authors of these studies have religious orientations," he says, "and that clouds the motivations." Some 74 percent of the women who participated in Frank-Herrmann's study, which will be published in the journal Human Reproduction, listed their religion as Roman Catholic, a faith for which this is the only church sanctioned method of family planning.

But Suzanne Parenteau-Carreau, a researcher and an advisor at Serena, a Canadian volunteer organization devoted to teaching couples how to practice the method, disagrees. She says that although religion was the early impetus, couples who practice STM are now seeking "natural'' birth control.

"Now it's more and more from a natural motivation; to be closer to nature," she says. "We often say it's people who like camping, bicycling, outdoor exercise—people who want healthy food and healthy natural family planning."

But that notion irks Grimes, who insists it is misleading. "I chafe at the term 'natural family planning,'" he says. "For many couples this is highly unnatural. 'Natural' is methods that you don't have to think about, that allow you to be spontaneous…. STM is very unpopular, hard to use, and has a poor success rate in average couples. Most people aren't willing to put up with it."

Frank-Herrmann acknowledges that one U.S. study conducted in 1980 in Los Angeles had a 90 percent dropout rate after less than two years.

But Grimes concedes that STM has its advantages, chief among them that "it is cheap, safe and approved by the Roman Catholic Church…. It is a reasonable part of the mix of contraceptive methods, it's just that for most people it's not an acceptable method."

Whether or not this method will ever gain ground on other contraceptive methods—the most recent data available indicate that less than 1 percent of women who use birth control in the U.S. use any method of periodic abstinence—all observers agreed that STM can only work for couples who stick to the plan 100 percent.

"It's not for everybody," Frank-Herrmann notes, "but there is a group of women who are interested in this method, and I think we should offer it to them."

The bottom line: all contraceptive methods have their drawbacks, including the potential of passing along the HIV virus and sexually transmitted diseases best prevented by condoms. Ultimately, Grimes says, "the best method for a couple to use is whatever they want. It's counterproductive to try to steer people to one thing or another."