Human Reproduction, Vol. 17, No. 7, 1925-1926,
July 2002
© 2002 European Society of Human Reproduction and Embryology
TwoDay Algorithm in predicting fertile time
2 Biostatistics Branch, MD A3-03, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA 3 Department of Statistics, University of Padua, Padua, Italy
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Dear Sir,
There are a number of factors that could have led to the higher estimated first-year pregnancy rate for the TwoDay Algorithm compared with the Billings Ovulation Method (BOM).
Focusing firstly on the possibilities raised by Dr. Mikolajczyk, these are: (i) days of abstinence: the TwoDay algorithm requires abstinence an average of 9 days per cycle (Sinai et al., 1999
), while the BOM requires abstinence during approximately half the days in each cycle (Trussell and Grummer-Strawn, 1991). Requiring more abstinence days tends to improve perfect-use effectiveness, while decreasing typical-use effectiveness; (ii) perception of secretions: ranking of cervical mucus symptoms depends for any method on the quality of the teaching provided and the perception of the women, both of which could have varied somewhat between the two studies. A formal comparison is not possible based on the available data; (iii) systematic under-reporting (secretions): there may have been less attention to observation and interpretation on days after a definitive peak had been identified in the Colombo and Masarotto study (Colombo and Masarotto, 2000). However, we have no evidence of systematic under-reporting by the European study participants, and the WHO data may also have contained reporting and classification errors (World Health Organization, 1981); (iv) statistical uncertainty: the lower confidence limit on the estimate reported in Dunson et al. was close to the Trussell and Grummer-Strawn estimate for the BOM (Dunson et al., 2001); (v) population fecundability: the women in the European database could have had higher fecundability on average (though we have no evidence of this); (vi) method of estimation: the Dunson et al. estimate was for a couple of average fecundability, while the Trussell and Grummer-Strawn estimate was based on an average across cycles at risk. Cycle-based estimates are biased downwards when couples of relatively low fertility contribute more cycles to the database; (vii) intercourse behaviour: the Dunson et al. estimate is relevant for cycles with intercourse on 25% of days classified as infertile, with no intercourse on other days. The Trussell and Grummer-Strawn estimate is for all cycles with perfect use. A low intercourse frequency within the perfect-use cycles of the WHO study for days classified as potentially fertile would lead to a low perfect-use pregnancy rate. Due to under-reporting of intercourse in the WHO study, it is not possible to evaluate this; (viii) systematic under-reporting (intercourse): as noted by Trussell and Grummer-Strawn, there was clear under-reporting of intercourse acts in the WHO study. With under-reporting less likely in pregnancy cycles because those were examined very carefully, relatively few pregnancy cycles would be classified as perfect-use, resulting in under-estimation of the perfect-use pregnancy rate. A similar bias is unlikely for the Colombo and Masarotto data, because couples were trained to collect intercourse data daily and conception and non-conception cycles were assessed by the same protocol.
Although all of the above factors could have played a role, the systematic under-reporting of intercourse in the WHO study seems likely to be a major contributor.
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3 To whom correspondence should be addressed: E-mail: dunson1{at}niehs.nih.gov
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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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