Human Reproduction, Vol. 17, No. 7, 1925,
July 2002
© 2002 European Society of Human Reproduction and Embryology
TwoDay Algorithm in predicting fertile time
1 Schenkendorfstr. 24, 39108 Magdeburg, Germany
Correspondence: E-mail: mikolajczyk723{at}compuserve.de
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Dear Sir,
Dunson et al. analysed the sensitivity of the TwoDay Algorithm in predicting fertile time (Dunson et al., 2001
) based on data from a recently reported European multicentre study (Colombo and Masarotto, 2000). They calculated a `theoretical' perfect-use first-year pregnancy rate for the TwoDay Algorithm of 8%. For another family planning method based on the mucorrhoea—the Billings Ovulation Method (BOM)—a perfect-use rate of 3% is assumed (Trussell and Grummer-Strawn, 1991; Trussell, 1998). Dunson et al. do not provide sufficient data for the understanding of why there is this difference between these two cervical secretion-based methods of family planning. Several possibilities could be considered: (i) the BOM and the TwoDay Algorithm have different rules for determining infertility: the higher rate of pregnancy for the TwoDay method could have accumulated where it assumes the end of fertility one day earlier than in the BOM method (Sinai et al., 1999); (ii) there might be a less accurate perception of cervical secretions by the European participants, as opposed to the documented better performance for BOM users in developing countries (World Health Organization, 1981) and (iii) the European study could have a systematic under-reporting of cervical secretions since the participants relied additionally on another biological indicator of fertility by monitoring their basal body temperature.
The authors could provide some answers to these possibilities by providing more detailed descriptions of the conception cycles (from the primary data charts) when the TwoDay Algorithm did not predict fertility. Another possibility would be to re-analyse the data with a `ThreeDay' Algorithm in which 3 days of no cervical secretions would be required before assuming infertility.
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Colombo, B. and Masarotto, G. (2000). Daily fecundability: first results from a new data base. Demographic research, 3, 5.
Dunson, D.B., Sinai, I. and Colombo, B. (2001). The relationship between cervical secretions and the daily probabilities of pregnancy: effectiveness of the TwoDay Algorithm. Hum. Reprod., 16, 2278–2282.
Sinai, I., Jennings, V. and Arevalo, M. (1999). The TwoDay Algorithm: a new algorithm to identify the fertile time of the menstrual cycle. Contraception, 60, 65–70.[Web of Science][Medline]
Trussell, J. (1998). Contraceptive efficacy. In Hatcher, R. (ed.) Contraceptive Technology. Ardent Media, New York. USA
Trussell, J. and Grummer-Strawn, L. (1991). Further analysis of contraceptive failure of the ovulation method. Am. J. Obstet. Gynecol., 165, 2054–2059.[Web of Science][Medline]
World Health Organization (1981). A prospective multicentre trial of the ovulation method of natural family planning. II. The effectiveness phase. Fertil. Steril., 36, 591–598.[Web of Science][Medline]
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