Fecundability - the monthly probability of conception - incorporates both physiological and behavioural elements that vary across a woman’s reproductive life span. How much of the variation in fecundability during the reproductive period can be attributed to age-related changes in physiology, and how much to variation in coital frequency? We use a mathematical model of fecundability to consider this question. Intra-uterine mortality has an important effect on fecundability: effective fecundability (the likelihood of a conception that results in a live birth) is less than half of total fecundability (the likelihood of any conception) at nearly all ages when coital frequency is held constant. The change in effective fecundability with increasing coital frequency is non-linear: it declines with increasing frequency. At all coital frequencies, the effects of increasing physiological age are greatest at the youngest and oldest reproductive ages, while between the ages of 20 and 30 physiological change has only a small impact on effective fecundability.
All Science Journal Classification (ASJC) codes