A natural family planning method is as effective as the contraceptive pill, German research suggests.
The method involves monitoring body temperature
The symptothermal method (STM) assesses fertility levels during the monthly cycle by measuring body temperature, and observing cervical secretions.
The Human Reproduction study found using STM correctly led to a rate of 0.4 pregnancies per 100 women per year.
UK experts said natural family planning was effective - provided it was taught properly and carried out correctly.
A University of Heidelberg team assessed STM in a study of 900 women.
The lowest pregnancy rate was found among women who abstained from sex during their most fertile period, as defined by STM.
Start of fertile period:
The first fertile day is when the woman first identifies either: 1) first appearance or change of appearance of cervical secretion, or 2) the sixth day of the cycle.
After 12 cycles, this second guideline is replaced by a calculation that subtracts seven days from the earliest day to show a temperature rise in the preceding 12 cycles.
End of fertile period:
The fertile phase ends after the woman has identified: 1) the evening of the third day after the cervical secretion peak day, and 2) the evening when the woman measures the third higher temperature reading, with all three being higher than the previous six readings and the last one being 0.2 degrees Celsius higher than the previous six.
Among those who used a barrier method during this time, such as a condom, the pregnancy rate rose to 0.6 pregnancies per 100 women per year.
Even among those women who had unprotected sex during the fertile period, the pregnancy rate was only 7.5 pregnancies per 100 women per year - around a quarter of the rate one would usually expect.
The researchers believe this was because the women probably only had unprotected sex around the boundaries of their most fertile period.
For a contraceptive method to be rated as highly effective it should lead to less than one pregnancy per 100 women per year when used correctly.
Lead researcher Dr Petra Frank-Herrmann said: "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."
CHANGE IN CERVICAL SECRETION
The amount of the hormones oestrogen and progesterone varies in the menstrual cycle, which alters the quantity and the consistency of cervical mucus
Just before ovulation the secretions become clearer, wetter, stretchy and slippery, like raw egg white
STM is governed by a precise set of rules, which do take some time to learn.
Some studies have suggested that a woman's libido is higher during her fertile period, and this could be one of the reasons why natural family planning methods have had a reputation for being less effective than other methods of family planning.
However, Prof Frank-Herrmann said: "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."
Toni Belfield, of the Family Planning Association (FPA), said: "Natural family planning when taught properly and carried out correctly is a highly effective method of family planning.
"Research has always shown that combining two or more fertility indicators is more effective than relying on a single fertility indicator.
"FPA has always provided full information about natural family planning as this is an important contraceptive choice for women to know about and use."