How the breast changes in pregnancy

During pregnancy the breast does not just grow: it remodels from the inside. The work is run by three hormones acting together, estrogen, progesterone, and prolactin.

In the first trimester the job is plumbing. The ducts, the tubes that milk will later travel through, lengthen and branch, and alveoli start to bud at their tips: small sacs that are the milk factories. It is a tree that forms its clusters long before the harvest.

In the second trimester prolactin steps in and changes the cells lining those sacs, which become lactocytes, the cells that actually make milk. That is when colostrum, the first milk, begins to accumulate.

All of this shows on the outside. The surface veins dilate, and the areola, the dark circle around the nipple, gets bigger and darker.

And even with the factory ready, copious milk does not come in before delivery. The progesterone of pregnancy keeps prolactin restrained; when the placenta is delivered that progesterone drops sharply, the brake comes off, and copious production begins.