Final answer:
Estrogen is not responsible for the secretion of prolactin during pregnancy; this is instead the function of the anterior pituitary gland, which is inhibited by estrogen, progesterone, and other hormones until after the placenta is expelled.
Step-by-step explanation:
The mechanism NOT attributable to estrogen during pregnancy is the secretion of prolactin. Although estrogen stimulates many changes during pregnancy, such as promoting fetal viability, growth of uterine smooth muscle cells, and development and growth of the mammary tissue, it is not responsible for prolactin secretion.
Prolactin, which begins to increase in concentration around the fifth week of pregnancy, is released by the anterior pituitary gland. Levels of prolactin become high enough to initiate milk production late in pregnancy, but estrogen, progesterone, and placental hormones inhibit this until after the placenta is expelled.
Therefore, the secretion of prolactin is not maintained by estrogen during pregnancy and preparation for delivery; instead, it is another critical hormone involved in the process.