<P> Galactopoiesis is the maintenance of milk production . This stage requires prolactin . Oxytocin is critical for the milk let - down reflex in response to suckling . Galactorrhea is milk production unrelated to nursing . It can occur in males and females of many mammal species as result of hormonal imbalances such as hyperprolactinaemia . </P> <P> The chief function of lactation is to provide nutrition and immune protection to the young after birth . In almost all mammals, lactation induces a period of infertility (in humans, lactational amenorrhea), which serves to provide the optimal birth spacing for survival of the offspring . </P> <P> From the eighteenth week of pregnancy (the second and third trimesters), a woman's body produces hormones that stimulate the growth of the milk duct system in the breasts: </P> <Ul> <Li> Progesterone influences the growth in size of alveoli and lobes; high levels of progesterone inhibit lactation before birth . Progesterone levels drop after birth; this triggers the onset of copious milk production . </Li> <Li> Estrogen stimulates the milk duct system to grow and differentiate . Like progesterone, high levels of estrogen also inhibit lactation . Estrogen levels also drop at delivery and remain low for the first several months of breastfeeding . Breastfeeding mothers should avoid estrogen - based birth control methods, as a spike in estrogen levels may reduce a mother's milk supply . </Li> <Li> Prolactin contributes to the increased growth and differentiation of the alveoli, and also influences differentiation of ductal structures . High levels of prolactin during pregnancy and breastfeeding also increase insulin resistance, increase growth factor levels (IGF - 1) and modify lipid metabolism in preparation for breastfeeding . During lactation, prolactin is the main factor maintaining tight junctions of the ductal epithelium and regulating milk production through osmotic balance . </Li> <Li> Human placental lactogen (HPL)--from the second month of pregnancy, the placenta releases large amounts of HPL . This hormone is closely associated with prolactin and appears to be instrumental in breast, nipple, and areola growth before birth . </Li> <Li> Follicle stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (hCG), through control of estrogen and progesterone production, and also, by extension, prolactin and growth hormone production, are essential . </Li> <Li> Growth hormone (GH) is structurally very similar to prolactin and independently contributes to its galactopoiesis . </Li> <Li> Adrenocorticotropic hormone (ACTH) and glucocorticoids such as cortisol have an important lactation inducing function in several animal species, including humans . Glucocorticoids play a complex regulating role in the maintenance of tight junctions . </Li> <Li> Thyroid - stimulating hormone (TSH) and thyrotropin - releasing hormone (TRH) are very important galactopoietic hormones whose levels are naturally increased during pregnancy . </Li> <Li> Oxytocin contracts the smooth muscle of the uterus during and after birth, and during orgasm (s). After birth, oxytocin contracts the smooth muscle layer of band - like cells surrounding the alveoli to squeeze the newly produced milk into the duct system . Oxytocin is necessary for the milk ejection reflex, or let - down, in response to suckling, to occur . </Li> </Ul>

When do mammary glands produce milk during pregnancy