Advances in human reproductive ecology.

Abstract

Human reproductive ecology pertains to reproduction biology and changes due to environmental influences. The research literature relies on clinical, epidemiological, and demographic analysis. The emphasis is on normal, nonpathological states and a broad range of ecological conditions. This review focused on the importance of age and energetic stress from ecological conditions rather than dieting or self-directed exercise in changing female fecundity. The literature on male reproductive ecology is still small but growing. J.W. Wood provided a comprehensive overview of the field. Natural fertility, as defined by Henry, is the lack of parity-specific fertility limitation. There is evidence that fertility can vary widely in natural fertility populations. There are consistent age patterns among different natural fertility populations. Doring found that there was higher frequency of anovulatory and luteal insufficiency in cycles during perimenarche and perimenopausal periods. Infertility studies have shown declines in pregnancy rates in women over the age of 30 years. Ovum donation evaluations have found both uterine age and ovarian and oocyte age to be related to the probability of a successful pregnancy. Basal follicle stimulating hormone and the endometrial thickness are important predictors of ovarian capacity and related to age and declining fecundity. Much of the literature on fecundity is derived from women with impaired reproductive physiology. In Lipson and Ellison's study of healthy women, average follicular and average luteal estradiol values declined with increasing subject age. Low follicular levels were correlated with smaller follicular size, low oocyte fertilizability, reduced endometrial thickness, and low pregnancy rates. Comparisons across populations have shown that populations experience declines in luteal function with age, but levels of luteal functions varied widely. Chronic conditions which slow growth and delay reproductive maturation may impact on lower ovarian function throughout adult life. There is a range of ovarian function along a continuum due to energetic stress. Evidence from the Lese in Zaire, the Tamang of Nepal, and Polish farm women outside Crakow suggest that workload affects ovarian function. Luteal function and ovulatory frequency is lower when women are losing weight. Among the Tamang losing weight between seasons there was evidence of lower ovarian function during the monsoon season. Polish farm women who work very hard in summer had lower ovarian function. The effect of lactation on amenorrhea appears to be due to the energetic stress on the mother in the intensity and duration of suckling. Women in poorer nutritional status may require more intense suckling. Seasonality of energy balance may be related to seasonality of female fecundity and conceptions.