Analysing cervical secretion and time to pregnancy data obtained from a large multinational European database - the European Study of Daily Fecundability - they were able to demonstrate that intercourse is unlikely to result in a conception if vaginal dampness is not noticeable on that day or the day before.
All a woman has to do is to notice when she has any vaginal dampness, not associated with menstruation, intercourse or disease. Women wishing to avoid pregnancy should avoid unprotected intercourse unless they have not had vaginal dampness for 2 days.
This algorithm, which was developed by the Institute for Reproductive Health at Georgetown University, is called the TwoDay method. The research teams used their data to analyse the relationship between the presence of noticeable secretions and the daily probabilities of pregnancy in cycles when intercourse was on a given day relative to the identified ovulation day. The TwoDay method differs from other symptom-based natural family planning methods in that it is not necessary to keep detailed records of cervical mucus characteristics and basal body temperature. This simple algorithm may outperform expensive urinary kits, which can miss the majority of the fertile interval that occurs one or more days prior to ovulation.
Dr David Dunson of the Biostatistics Branch of the National Institute of Environmental Health Sciences in North Carolina said: "This method is effective, both in identifying the fertile days of the cycle and in predicting days within that fertile interval that have a high pregnancy rate. It's the first direct evidence that cervical secretions are associated with higher fecundability within the fertile window. For couples of normal fertility having intercourse two days prior to ovulation on the most fertile day of the cycle, the probability of pregnancy is essentially doubled from 0.18 (18%) if secretions have not been noticed in the last two days to 0.33 (one third) if secretions have been noticed. A normal couple who abstains from intercourse during the days classified as fertile by our system would have around an 8% chance of becoming pregnant within a year of frequent intercourse - compared with a 97% chance for a couple not following our system."
*The relationship between cervical secretions and the daily probabilities of pregnancy: effectiveness of the TwoDay Algorithm. Human Reproduction Vol. 16. No. 11. pp 2278-2282. D.B. Dunson, I.Sinai, B Columbo. Biostatistics Branch, National Institute of Environmental Health Sciences, North Carolina; Institute for Reproductive Health, Georgetown University, Washington DC, Department of Statistics, University of Padua.