The researchers say that reliable independent data on most of the systems used are still not available and that monitors differ enormously in price and effectiveness.
About 10-15 per cent (but with a very high estimated number of unknown cases) of women of reproductive age in Europe use variations of natural family planning methods and an estimated 30-40 per cent rely on some type of menstrual cycle monitor to detect the fertile 'window' in their menstrual cycle, usually for contraceptive purposes, but sometimes to optimise their chances of conception.
"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," said lead investigator emeritus Professor Guenter Freundl, gynaecological endocrinologist and senior partner at the Department of Reproductive Medicine and Gynaecological Endocrinology at Staedtische Kliniken Düsseldorf gGmbH, Frauenklinik Benrath, Düsseldorf.
The researchers have developed a method of testing the monitors by devising a QI (quality index). The days predicted as fertile by each of the systems were compared with the fertile time revealed by ultrasound scans and the detection of the surge of LH (luteinizing hormone) in the urine and basic daily conception probabilities. The resulting attributed QI ranges from zero to one, with small values indicating a good method for preventing a pregnancy and a value close to one being close to 'no method at all'. Six monitors were tested plus the symptothermal method (STM) of natural family planning (NFP) relying on a woman's own observations.
The findings from the 62 women involved in this study have already led the researchers to say that systems that do not meet the basic QI standard, which they have set at about 0.5, should not be offered to women and are not worth further investigations as contraceptive methods because they are likely to have an unacceptable failure rate in a prospective study of efficacy.
This 0.5 cut-off would rule out from further contraceptive testing three monitors - systems using a mini-microscope, which rely on the user recognising 'ferning' patterns from small samples of saliva or cervical mucus. Ferning patterns indicate fertility, so an absence of 'ferning' indicates a non-fertile day. But, the three systems using this method registered false negatives of 51.8%, 58% and 73.4% respectively for the women who tried one of these methods. This meant that even the most accurate was wrong at least half the time that it indicated that the woman was in a non-fertile part of her cycle while the worst was wrong nearly three-quarters of the time.
By contrast, the symptothermal method, which relies on a woman's own observation of two indicators of fertility - changes in her basal body temperature and in her cervical mucus patterns, plus some calculation rules - gave no false negatives for the 15 women using this method.
Three methods also applying the rules of the body temperature method but involving a mini-computer to record and store the data also performed reasonably well with 1.7%, 4.7% and 7.5% false negatives for the women who tried one of these methods. A seventh method, involving a monitor that tested hormone levels in the urine, recorded 20.8% false negatives.
"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. The temperature computers scored at the upper range of reliability and the hormonal computer in the medium range. But, the mini-microscopes had a very low estimated efficacy," said Professor Freundl.
"Before undertaking a full prospective clinical trial with all the effort and expense involved, it was essential to perform a primary efficacy estimation analysis and this is what our study has done. To our knowledge there has been no similar approach to compare the different cycle monitors or fertility prediction methods for contraception," he said. "The next step is to move to full clinical trials of those methods that met our basic QI standard."
 Estimated maximum failure rates of cycle monitors using daily conception probabilities in the menstrual cycle. Human Reproduction. Vol.18. No. 12. pp 2628-2633.
 The investigating team involved researchers from: Staedtische Kliniken, Düsseldorf, the University of Heidelberg and Stiftung Warentest, Berlin.
 Sixty-two women, with a mean age of 31, contributed a total of 122 menstrual cycles to the study. Every woman was asked to test two different types of monitoring to minimise a probable bias in interpretation of partly subjective methods. They tested the system for 7-8 cycles with the first six being learning cycles, which was necessary to set up the database for the temperature computers and the hormone monitor.
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