I would have answered an astounding NO.

But apparently...YES. If used correctly -- as emphasized in a report published online February 21 in Human Reproduction.

The natural family planning being described in the report is the symptothermal method (STM) - a form of natural family planning (NFP) that enables couples to identify accurately the time of the woman's fertile phase using two indicators: measuring the woman's temperature and observing cervical secretions.

According to lead author of the report, Petra Frank-Herrmann, assistant professor and managing director of the natural fertility section in the Department of Gynaecological Endocrinology at the University of Heidelberg, Germany:

"For a contraceptive method to be rated as highly efficient as the hormonal pill, there should be less than one pregnancy per 100 women per year when the method is used correctly.

The pregnancy rate for women who used the STM method correctly in our study was 0.4%, which can be interpreted as one pregnancy occurring per 250 women per year.

Therefore, we maintain that the effectiveness of STM is comparable to the effectiveness of modern contraceptive methods such as oral contraceptives, and is an effective and acceptable method of family planning."

[The researchers found that if the couples then either abstained from sex or used a barrier method during the fertile period, the rate of unplanned pregnancies per year was 0.4% and 0.6% respectively.]

The study - the largest prospective study of STM - was conducted by the German Natural Family Planning Study Centre between 1985 and 2005 which involved 900 women who provided data on 17,638 cycles to Prof Frank-Herrmann and her colleagues.

Find more details from the full report.