Go read the Disclaimer again. I am not a doctor. This is not medical advice. Seriously.
Prolactin is a hormone which is highly elevated during pregnancy. It does not cause you to make milk throughout the pregnancy in part because other hormones are also highly elevated, and they work in opposition to prolactin. (This is related to why estrogen based birth control negatively affects milk supply.) Prolactin is produced by the pituitary, not the ovaries, so even women who have had complete hysterectomies (and men) can produce milk. Prolactin is inhibited by dopamine (or something plus dopamine), so people treated for depression often experience lactation.
Prolactin production goes up in response to nipple stimulation (up to 10-20 times in response to suckling). It is higher at night, like oxytocin, another pituitary hormone, but with a different patten: it begins to rise in the evening and peaks in the early morning. This is yet another reason for sleeping with or near your baby.
Prolactin is very protective against the effects of stress.
Prolactin prevents the release of luteinizing hormone from the pituitary gland. Luteinizing hormone causes an egg to be released (ovulation) and the luteal phase to begin. If prolactin levels drop and stay low (a long interval between breastfeeding), an egg can slip out. But even if ovulation occurs, and the resulting egg is fertilized, regular, high levels of prolactin from frequent, intensive breastfeeding will make successful implantation less likely (about 30% reduction in fertility associated with breastfeeding, even after menstruation has returned), by further interfering with estrogen and progesterone, leading to a short luteal phase (less than the ten days necessary for successful implantation).
Prolactin is really effective at preventing pregnancy. High prolactin levels from other causes (such as a tumor in the pituitary gland, and a long list of other things including emotional or physical stress, exercise or a lot of protein) accounts for a significant number of women who have irregular ovulation and go see a fertility specialist in an effort to get pregnant.
A general discussion of postpartum hormones
Copyright 2006 by Rebecca Allen.
Created February 7, 2006 Updated March 8, 2006