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  • Title: A probability model for number of conceptions when sterility is age dependent.
    Author: Srivastava U, Singh KK.
    Journal: Janasamkhya; 1989 Jun; 7(1):59-70. PubMed ID: 12282982.
    Abstract:
    Probability models that seek to describe the distribution of conceptions during an time span that is a distant point from marriage must take into account the age-dependent onset of secondary sterility. The model developed in this paper is intended for the analysis of conceptions occurring during the later stages of the reproductive span when the incidence of secondary sterility is steadily increasing. The model's parameters are the rest period associated with each live birth (h), the probability that the female is fecund at the start of the observational period (a1), the probability that the female becomes sterile during the study period (a2), and the rate of conception (m). For illustration purposes, this model was applied to data from a survey conducted in 1978 in rural Varanasi on the number of births in the preceding 7 years to women 35-40 and 40-45 years of age. Given an estimated value of a1 of 0.949 and of a2 of 0.658 for women 35-40 years of age, 63.4% of women 37.5 years (average age) are fecund. Similarly, if a1 is estimated at 0.722 and a2 at 0.525 for women 40-45 years old, 37.9% of women are fecund at the average age of 42.5 years. m is estimated at 0.501 for the younger age group and 0.394 for the older age group, redlecting th enumerous biological, behavioral, and environmental factors that lead to a decrease in fecundity after age 35 years. The difference between estimates of conceptions obtained from this model and from a model that assumed no change in fecundity during the observational period were not significant, but this is due to the fact that the latter and not the former model incorporated fetal wastage. Use of the model outlined in this paper is recommended for estimations of conceptions to women over 35 years of age.
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