Endocrinology of reproduction half of pregnancy, progesterone and prolactin prepare the mammary glands for lactation. Estrogens and progesterone promote mammary epithelial cell proliferation resulting in the formation of the primary and secondary ductal structure. Progesterone induces formation of tertiary side-branches in the mammary glands during puberty and during the luteal phase of the menstrual cycle upon which lobuloalveolar structures form under the influence of prolactin. Prolactin stimulates lactogenesis. hCG appears to be soporific during pregnancy; levels of hCG correlate with sleep changes during pregnancy, and administration of hCG increases sleep in rats likely via neuronal LHCGR. Endocrinology of reproduction Hormonal regulation occurs at
Estrous cycle that releases sex hormones including estrogens and progesterone. However, species vary significantly in the detailed functioning. One difference is that animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead. Another difference is sexual activity. In species with estrous cycles, females are generally only sexually active during the estrus (oestrus) phase of their cycle . This is also referred to as being "in heat". In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even