Final answer:
The hormone primarily responsible for the growth of mammary gland ducts during pregnancy is estrogen, while both estrogen and progesterone are important for breast development and maintaining the pregnancy.
Step-by-step explanation:
The hormone primarily responsible for the growth of mammary gland ducts during pregnancy is estrogen. Estrogen stimulates tissue growth in the mother and developing fetus and has a crucial role in preparing the mammary glands for lactation. Progesterone supports pregnancy by suppressing uterine contractions and preventing the development of new ovarian follicles. Both hormones are essential for pregnancy and breast development, but it is estrogen that specifically promotes the growth of mammary gland ducts.During pregnancy, the anterior pituitary gland also increases the production of prolactin, which contributes to the development of the mammary glands and prepares them for milk production after childbirth. However, the initiation of milk production is inhibited by high levels of estrogen and progesterone until after the birth, when the expulsion of the placenta lifts this inhibition.