Prolactin. Guide. A prolactin test measures the level of the hormone prolactin, which is made by the pituitary gland, in your blood. Pregnant women have high levels of prolactin, which helps make breast milk.During pregnancy, prolactin levels increase by 10 to 20 times.After the baby is born, prolactin stays high if you are breast-feeding. In women who do not breast-feed, prolactin levels return to normal soon after they give birth.regnant women have high levels of prolactin, which helps make breast milk. During pregnancy, prolactin levels increase by 10 to 20 times. After the baby is born, prolactin stays high if you are breast-feeding. In women who do not breast-feed, prolactin levels return to normal soon after they give birth.