Researchers have identified the natural family planning method - employing two indicators of the fertile phase in a woman's menstrual cycle: her temperature and cervical secretions - to be as effective as the contraceptive pills.

If during the fertile phase the couples either abstained or used a barrier method, the level of unplanned pregnancies per year dropped to 0.4% and 0.6%, respectively.

From 900 women subjects, only 1.8 % became unintentionally pregnant. "For a contraceptive method to be rated as highly efficient as the hormonal pill, there should be less than 1 % pregnancy per year when the method is used correctly. Therefore, we maintain that the effectiveness of symptothermal method (STM) is comparable to the effectiveness of modern contraceptive methods such as oral contraceptives, and is an effective and acceptable method of family planning", said lead researcher Petra Frank-Herrmann, assistant professor and managing director of the natural fertility section in the Department of Gynecological Endocrinology at the University of Heidelberg, Germany.

The 900 women, who were part of a research made by the German Natural Family Planning study center between 1985 and 2005, provided data on 17,638 cycles.

STM detects the beginning and end of a woman's fertile phase through two measurements (body temperature and cervical secretions), as a double-check system. The first fertile day installs with the first appearance or change of appearance of the cervical secretion, usually in the sixth day of the cycle. After 12 cycles, a six-day period must be recounted by a calculation that subtracts seven days from the earliest day to show a temperature rise in the preceding 12 cycles.

The fertile phase ends after the evening of the third day after the cervical secretion peak day, usually coinciding with the third higher temperature, all three being higher than the previous six and the last one 0.2 degrees C higher than the previous six. "Learning STM is usually no problem. However, in contrast to the oral contraceptive pill or other family planning methods, STM needs more engagement and time to learn it", said Frank-Herrmann.

Each month, the volunteers sent data with their cycles, their observations of temperature and cervical secretions, and recorded their sexual behavior and family planning intentions for the next cycle. 322 women employed only STM and 509 used STM combined with barriers during the fertile period. But the 0.4 % and 0.6 % rates rose to 7.5 % when the couples had unprotected sex during the fertile periods.

The drop-out rate from employing STM, due to dissatisfaction or difficulties with the method was 9.2 %, while other methods can have drop-out rates of 30%. "This demonstrates a fairly good acceptability for this particular FAB (fertility awareness based) method," said Frank-Herrmann.

The relatively low rate of unintended pregnancies (7.5%) among women who practiced unprotected sex during their fertile time puzzled the researchers. "If people are trying for pregnancy you expect a pregnancy rate of 28% per cycle. Therefore, we think that some of the couples were practising conscious, intelligent risk-taking, and were having no unprotected sex during the few highly fertile days, but had unprotected intercourse on the days at the margins of the fertile time when the risk of pregnancy was lower."

Some researches pointed that women's sex drive would be higher during their fertile phase and this makes the natural method not very alluring. "There are studies that suggest that this is only the case for a small proportion of women, and that, in fact, women also identify other parts of their cycle with increased sexual desire. Most women who use FAB do not find this a problem. It's possible that the increased libido may be one of the reasons that some of the couples in our study used a barrier, such as a condom, in the fertile phase." said Frank-Herrmann.