With this discussion. it is clear that a complex array of factors related to heat stress affects embryonic survival. The most significant of these may be thermal sensitivity of embryos during very early development and the uterine environment as affected by circulating concentrations of progesterone after the embryo reaches the uterus. To date, management strategies have not been developed that result in a marked improvement in summer fertility. Accordingly, most dairy managers minimize the number of cows bred during the most stressful periods