Uterine blood flow may be related to this response. Reduced uterine blood flow has been observed during heat stress, especially during later stages of pregnancy. Occurrence of this during early pregnancy would probably slow embryonic growth. It has been postulated that delayed embryonic growth could lead to deficient production of interferon-t, the antiluteolytic protein synthesized by bovine and ovine embryos by days 13-15. Interferon-t causes reduced synthesis and release of prostaglandin F2a, by the uterus, which permits continued function of the corpus luteum and maintenance of pregnancy. If heat stress causes a deficiency of interferon-t, this would probably lead to lower progesterone through reduced function of the corpus luteum and lower fertility. Lower progesterone has not been observed in some summer studies. Therefore. other factors may be interacting to affect the response of the corpus luteum to heat stress.