German researchers have found that natural methods of family planning like the symptothermal method (STM) are as effective as modern contraceptives. This finding challenges the common conception that natural methods of birth control are the least effective among all the family planning methods that are available today.
In 1985, scientists at University of Heidelberg started a prospective, longitudinal study of 900 women using the symptothermal method for family planning (not compared to another population). The symptothermal method requires a woman to define her fertile interval during the monthly cycle through measuring body temperature and observing cervical secretions. 
It was found that the lowest pregnancy levels (an average 0.43 pregnancies per 100 women per year,  i.e. 13 cycles, with 95% confidence intervals of 0.05-1.55) were among women who abstained from sex contact or practiced non-penetrative sex during their most fertile period (as defined by the symptothermal method). The rate was slightly higher (0.59 pregnancies per 100 women per year) among women who used barrier methods of contraception (for example, a condom) during the same period.
These figures indicate that the symptothermal method might be reliable, but more research is needed to increase the statistical power of the data. Scientists consider a method of contraception to be highly effective if the pregnancy rate is less than one pregnancy per 100 women per year when used properly (the upper limit of confidence was 1.55 in this case, but the average 0.43).
The study reported a dropout rate of 9.2 per 100 woman. Study participants indicated that this was either due to the fact that they no longer wished to participate, or that they wanted to achieve pregnancy, for medical reasons, because they separated from their partner, etc.
Surprisingly enough, the rate of pregnancy among women who had sex during their fertile period turned out to be only 7.46 pregnancies per 100 women per year, although the rate was four times higher than what could be expected. The researchers say these women had sex on the boundaries of their fertile periods.
Taken together, these findings suggest that the symptothermal method of contraception is worthy of further research, as a possible future option for those who for some physical or religious reasons cannot use other family planning methods. 
Frank-Herrmann P, Heil J, Gnoth C, Toledo E, Baur S, Pyper C, Jenetzky E, Strowitzki T, Freundl G. The effectiveness of a fertility awareness based method to avoid pregnancy in relation to a couple's sexual behaviour during the fertile time: a prospective longitudinal study. Hum Reprod, 2007; Epub ahead of print: doi:10.1093/humrep/dem003. PubMed
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Natural methods of family planning under investigation