[Lotus]

During pregnancy, high circulating concentrations of estrogen increase prolactin levels by 10- to 20-fold.

I don't think that's how it goes.
During pregnancy, it's the placenta that raises progesterone sky high, to prevent a miscarriage. Progesterone=the pregnancy hormone. Now, progesterone from the placenta signals the pituitary to release more prolactin. Progesterone is the dominant hormone during pregnancy, and prolactin is secondary

Prolactin is stimulated by the baby breastfeeding, which also stops menstruation. When the baby is no longer breastfed, the menstrual cycle returns to normal. Here prolactin influences progesterone to also rise. Prolactin is the dominant hormone during breastfeed, and its function is breastfeeding. The reason for this is, the mother doesn't need to seek partners while nursing her child.

Both prolactin and progesterone lower estrogens. And estrogens lower both prolactin and progesterone.

If you meant [During pregnancy, high circulating concentrations of '''progesterone''' increase prolactin], that sounds more right, but I'm not sure by how much.

Excessive estrogen in circumstances of deficient progesterone induces a decrease in [estrogen] receptor sensitivity. One of progesterone's functions is to restore the normal sensitivity of estrogen receptors. When progesterone is restored, estrogen receptor sensitivity is restored also.

Clarified estrogen receptor. The rest is right.

That's directly from http://wikipedia.org/wiki/Prolactin


During pregnancy, high circulating concentrations of estrogen increase prolactin levels by 10- to 20-fold. However, at the same time, estrogen, as well as progesterone, inhibit the stimulatory effects of prolactin on milk production. It is the abrupt drop of estrogen and progesterone levels following delivery that allows prolactin — which temporarily remains high — to induce lactation.

After childbirth, prolactin levels fall as the internal stimulus for them is removed. Sucking by the baby on the nipple then promotes further prolactin release, maintaining the ability to lactate. The sucking activates mechanoreceptors in and around the nipple. These signals are carried by nerve fibers through the spinal cord to the hypothalamus, where changes in the electrical activity of neurons that regulate the pituitary gland cause increased prolactin secretion. The suckling stimulus also triggers the release of oxytocin from the posterior pituitary gland, which triggers milk let-down: Prolactin controls milk production (lactogenesis) but not the milk-ejection reflex; the rise in prolactin fills the breast with milk in preparation for the next feed.