To the Editor:
We congratulate Bilian et al. on an important contribution to the research informing day-specific probabilities of conception (1). However, we question their conclusion of an ethnic difference in fecundity. Over half of their female participants had a previous pregnancy (Table 1), and all of the men had clinically proven fertility, which selected for couples of substantially higher fecundity than the participants in the North Carolina Early Pregnancy Study (2). The selection of cycles with a detectable LH surge correlating to an expected luteal phase of at least 13 days may also further select cycles with higher fecundity. These considerations are probably sufficient to account for the greater day-specific probabilities of conception observed in their data compared to the North Carolina study. Of interest, the study reported a rate of spontaneous abortion of 10/601, or 1.7%. We wonder whether the authors have any possible explanation for this extraordinarily low rate.
We wish also to mention a few statistical concerns. It appears equation three should have P instead of 1-P on the left-hand side. Their application of the Barrett-Marshall model with a fixed intercept works well for one or two episodes of intercourse during the fertile time, but has some improbable results for some patterns of intercourse (for example, a probability of conception 0.26 if no intercourse occurs during the fecund window, and a probability of 0.15 if intercourse occurs daily from day -7 to day -4 inclusive).
We disagree that the urine LH surge is more a sensitive indicator of ovulation than the algorithm used by Wilcox et al. Further, formal adjustment for the variability in the timing of the LH surge would be expected to yield a somewhat narrower estimate of the fecund window (3).
Finally, because the estimation of emergency contraception effectiveness depends on the underlying fecundity of the population, the probabilities from this study should not be applied to postulate a higher effectiveness of emergency contraception (EC) in general (4,5). To use their own day-specific probabilities of conception to estimate the clinical effectiveness of EC, EC should be tested in a sample of women who have had intercourse with men of proven fertility, and have cycles with a normal LH surge and a normal expected length of the luteal phase.
Joseph B. Stanford, M.D., M.S.P.H.
Department of Family and Preventive Medicine
University of Utah
Salt Lake City, Utah
Rafael T. Mikolajczyk, M.D., M.S.
Department of Clinical Epidemiology
Bremen Institute for Prevention Research and Social Medicine
University of Bremen
David B. Dunson, Ph.D.
Department of Statistical Science
Durham, North Carolina
1. Bilian X, Heng Z, Shang-Chun W, et al. Conception probabilities at different days of menstrual cycle in Chinese women. Fertil Steril, epub July 2009.
2. Mikolajczyk RT, Stanford JB. Measuring fecundity with standardised estimates of expected pregnancies. Paediatr Perinat Epidemiol 2006;20 Suppl 1:43-50.
3. Dunson DB, Weinberg CR, Baird D, Kesner JS, Wilcox AJ. Assessing human fertility using several markers of ovulation. Stat Med 2001;20:965-8.
3. Mikolajczyk RT, Stanford JB. A new method for estimating the effectiveness of emergency contraception that accounts for variation in timing of ovulation and previous cycle length. Fertil Steril 2005;83:1764-70.
4, Stanford JB, Mikolajczyk RT. Methodological review of the effectiveness of emergency contraception. Current Women’s Health Reviews 2005;1:119-29.
Published online in Fertility and Sterility