This article sets forth a probability model to describe variation in the interval between marriage and 1st conception. The 1st conceptive delay parameter has utility for estimating human fertility factors such as fecundability, adolescent sterility, and premarital conceptions. Previous models analyze only the waiting times of women who had conceived by the survey point, thereby eliminating from the analysis those who had not conceived. The present model was applied to data on months required to conceive for the 1st time collected in the Menstrual and Reproductive History Study. The distribution was adjusted to include women with primary sterility. The proposed model provided a reasonable approximation to the real process.
This paper derives a probability model of the waiting time from marriage to 1st conception by using a mathematical function to show the variation of fecundability over a woman's reproductive span. The model is based on the assumptions that conception is a chance event, a woman begins her reproductive cycle after marrige in a fecundable and nonpregnant state, the age at marriage is a random variable with unknown distribution function, and all women are homogeneous with respect to fecundability. To test the validity of the model, it was fitted to the data of Singh. The proposed model described the observed data satisfactorily on the basis of age at marriage. Weaknesses in the fit of the data in some cells may reflect recall and adjustment errors. The advantage of this model over those that assume that the coition rate of couples is the same until conception is that it takes into account the effect of social customs regulating sexual activity after marriage.
