Declining fecundity and ovarian ageing in natural fertility populations

Worldwide, human fertility declines with increasing maternal age, after con traceptive-use patterns and behavioral factors are taken into consideration . Here, we summarize some of our theoretical and empirical work examining t he biological factors contributing to this age pattern of fertility. We und ertook an Ii month prospective endocrinological study in a natural fertilit y (non-contracepting) population (rural Bangladesh) to estimate the contrib utions of fetal loss and fecundability (the probability of conception) to d eclining fecundity with age. Prospective interviews and urine samples for p regnancy tests were collected twice weekly from up to 700 women. These data were used to test mathematical models of the underlying biological process es contributing to changing fecundability and fetal loss risk with maternal age. The results indicate that much of the decline in fecundity can be att ributed to an increasing risk of fetal loss with maternal age. Much of this fetal loss is due to chromosomal abnormalities-a result of ageing oocytes. Fecundability, on the other hand, does not begin to decline until the earl y 40s. We hypothesize that this is also a result of ageing at the ovarian l evel, namely follicular atresia, in the years just prior to menopause. The irregularity of menstrual cycles-longer cycles and increasingly variable ho rmonal patterns-at these ages may be a direct result of the small and rapid ly dwindling remaining pool of follicles. We present a simple mathematical model of this process, and some preliminary laboratory results that support the model. (C) 1998 Elsevier Science Ireland Ltd. AU rights reserved.