Why is prolactin high during pregnancy?

Bottom lineProlactin rises 10 to 20 times above baseline during pregnancy - roughly 80 to 400 ng/mL versus under about 25 when not pregnant - because it is building the milk-producing tissue in your breasts. This is normal and is not treated. Milk does not arrive during pregnancy because placental progesterone blocks prolactin from finishing the job; when the placenta is delivered progesterone crashes, the block lifts, and milk comes in on days 2 to 3. High prolactin also suppresses ovulation, which is why periods often pause while breastfeeding.

Because it is building your milk supply system. Prolactin rises steadily through pregnancy and reaches 10 to 20 times its baseline by the end of gestation - roughly 80 to 400 ng/mL, compared with under about 25 ng/mL when not pregnant. This is entirely normal and is not tested or treated.

What the rise is doing

Prolactin drives the growth of the milk-producing glandular tissue in your breasts and gets the machinery for making milk into place. It is a large part of why breasts change in size and sensitivity so early in pregnancy, often as one of the first noticeable signs.

Why milk doesn't arrive until after birth

The placenta produces high levels of estrogen and progesterone, and progesterone blocks prolactin from actually triggering milk production. You have the hormone and a breast that is ready, but the switch is held down.

Some women leak small amounts of thick, yellowish colostrum in the third trimester. That is normal and does not mean anything has gone wrong.

What flips the switch

When the placenta is delivered, progesterone crashes within hours. The block lifts, prolactin is left standing, and milk production begins - which is why milk "comes in" on days 2 to 3 rather than immediately. The delay is progesterone clearing your system, not a sign that anything is wrong.

After that, supply becomes demand-driven: each time the baby suckles, a pulse of prolactin follows.

High prolactin is also why periods often pause during breastfeeding - it suppresses ovulation. Read more in prolactin in women.

Femora helps you track the return of your cycle after birth, which can happen before your first period.