Final answer:
The placenta is the structure that secretes essential hormones, such as hCG, estrogens, and progesterone, taking over biochemical control of the mother's body during pregnancy. It plays a key role in maintaining the pregnancy and supporting fetal development, and it becomes the main endocrine organ as it progresses.
Step-by-step explanation:
The structure that secretes essential hormones and takes over a large measure of biochemical control of the mother's body during pregnancy is the placenta. The placenta begins to form shortly after the blastocyst embeds into the uterine lining. It secretes several hormones, including human chorionic gonadotropin (hCG), estrogens, and progesterone, which are crucial for maintaining the pregnancy and stimulating both maternal and fetal development.
These hormones mediate the transmission of maternal hormones into fetal blood and vice versa, thereby playing an indispensable role during pregnancy. During the initial weeks from the last menstrual period (LMP) pregnancy hormones are primarily produced by the corpus luteum. However as the placenta develops, it takes over as the endocrine organ of the pregnancy. The anterior pituitary also enlarges and increases the production of hormones such as thyrotropin, prolactin, and adrenocorticotropic hormone (ACTH), which contribute to the changes experienced during pregnancy. Progesterone, in particular, is essential as it prevents the development of new ovarian follicles and suppresses uterine contractility, protecting against premature labor.