Measuring fecundity with standardised estimates of expected pregnancies

Approaches to measuring fecundity include the assessment of time to pregnancy and day-specific probabilities of conception (daily fecundities) indexed to a day of ovulation. In this paper, we develop an additional approach of calculating expected pregnancies based on daily fecundities indexed to the last day of the menstrual cycle. Expected pregnancies can thus be calculated while controlling for frequency and timing of coitus. Comparing observed pregnancies with expected pregnancies allows for a standardised comparison of fecundity between studies or groups within studies, and can be used to assess the effects of categorical covariates on the woman or couple level, and also on the cycle level. This can be accomplished in a minimal data set that does not necessarily require hormonal measurement or the explicit identification of ovulation. We demonstrate this approach by examining the effects of age and parity on fecundity in a data set from women monitoring their fertility cycles with the Creighton Model FertilityCare System.