VARIATION IN LEVELS OF REPRODUCTIVE HORMONES

High levels of ovarian steroid hormones in menstrual cycles are crucial for successful pregnancy (Lipson and Ellison,1996; Venners et al., 2006) and, as such, are important determinants of female reproductive success and evolutionary fitness. However, there are substantial differences in mean levels of estradiol and progesterone between populations, among women within a single population, and among menstrual cycles of a single woman (Figure 19.1) (Ellison et al., 1993; Jasienska and Jasienski, 2008). For example, urban women in the United States have progesterone levels that are on average 65% higher than those of women from the Democratic Republic of Congo (Ellison et al., 1993). In a rural population from Poland, as much as 46% of the among-cycle variation in salivary progesterone is due to differences among individual women, while the remaining 54% of variation is due to differences among cycles of individual women (Jasienska and Jasienski, 2008). Such high intercycle variation is probably caused by a seasonality of agricultural workload and is much higher than in nonseasonal, industrial populations. However, even in urban women from the United States and the United Kingdom, where lifestyle is less influenced by seasons, progesterone levels vary from cycle to cycle (Lenton et al., 1983; Sukalich et al., 1994; Gann et al., 2001).

The present chapter reviews recent findings about variation in human female ovarian function, and more specifically, the levels of two primary female reproductive hormones: 17-β estradiol and progesterone.