Devices which help women predict the fertile days of their cycle may be giving false readings, say researchers.
The risk of unplanned pregnancy could rise
They have called for urgent research into some systems, which help couples follow "natural family planning" methods avoiding active contraception.
A variety of methods are used to spot changes which suggest a woman is approaching her "fertile window".
But German experts said, in the journal Human Reproduction, that using some, more than half of readings were wrong.
They say this could lead to unplanned pregnancies in the absence of any other methods of birth controls.
Professor Guenter Freundl, from the Department of Reproductive Medicine at the Staedtische Kliniken Dusseldorf, said: "If a couple are going to rely on these monitors, especially for contraception, then it is extremely important that they have reliable information about the effectiveness and accuracy of the different systems."
The findings from 62 women and their monitors in their preliminary study, he says, already mean that some do not meet a minimum quality standard and should not be offered to women.
These included systems involving a mini-microscope, which rely on the user recognising a particular pattern in small samples of their cervical mucus or saliva.
The three systems tested registered "false negative" rates - in which the woman believes she is not fertile that day, when in fact she is - in 51.8%, 58% and 73.4% of tests respectively.
Other methods were more reliable. One involving temperature tests in combination with changes in cervical mucus composition - often taught by instructors to women, gave no false negatives among the 15 women using the method.
Secretions become heavier and change in texture around ovulation - the time of the month when an egg is released ready for fertilisation.
Another body temperature method involving a mini-computer to record and store the data produced between 1.7% and 7.5% false negatives.
Finally, a urine hormone test managed 20.8% false negatives.
Professor Freundl said: "This meant that the symptothermal method of NFP that relies on a woman's own observations proved to be the most effective of the natural family planning methods.
"But, the mini-microscopes had a very low estimated efficacy."
The chief executive of the FPA, Anne Weyman, told BBC News Online that the study suggested that some natural family planning methods were extremely effective.
She said: "This study shows the most reliable means of natural family planning is the woman herself. This method is best taught by a specialist teacher and can be up to 98% effective if used according to the teaching instructions.
"Women who do not wish to use other methods can have confidence in natural family planning.
"Women can either plan or avoid a pregnancy if they correctly monitor their own bodies for indications of their fertile time."