Final answer:
At the end of the third month of pregnancy, the placenta starts to secrete essential hormones such as estrogens, progesterone, and hCG, which are crucial for maintaining the pregnancy and preparing for childbirth.
Step-by-step explanation:
Near the end of the third month of pregnancy, the placenta begins to secrete several vital hormones, including estrogens, progesterone, and human Chorionic Gonadotropin (hCG). These hormones play critical roles in maintaining and advancing the pregnancy. The corpus luteum, which initially secretes these hormones, is eventually replaced by the placenta. During weeks 12-17, as the corpus luteum degenerates, the placenta becomes the primary endocrine organ, taking over the production of these hormones. Estrogen, which is synthesized from weak androgens from both the maternal and fetal adrenal glands, ensures the progression of pregnancy, with levels rising throughout gestation. Progesterone suppresses the release of Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), inhibits uterine contractions to prevent preterm birth, and begins to decrease in late gestation to allow for labor. hCG is also produced, maintaining the corpus luteum earlier in the pregnancy and stimulating testosterone secretion in male fetuses. The placenta, hence, is a critical component in the exchange of nutrients, waste, and protective immunoglobulins from the mother to the fetus.