Final answer:
The secretion of hormones by the corpus luteum during pregnancy is sustained by human chorionic gonadotropin (hCG) until the placenta becomes mature enough to produce these hormones itself, usually around the 12th week of pregnancy.
Step-by-step explanation:
The secretion of estrogen and progesterone by the corpus luteum is maintained during pregnancy by human chorionic gonadotropin (hCG) until the corpus luteum degenerates and the placenta takes over secretion of these hormones.
This transition occurs because the corpus luteum, which forms in the ovary after ovulation, initially produces significant levels of estrogen and progesterone to maintain the endometrium and support a potential pregnancy. As the endometrium is maintained, these hormones particularly progesterone, inhibit the production of GnRH, FSH, and LH, thus preventing ovulation during pregnancy.
In the early stages of pregnancy, hCG, which is produced by the developing embryo and then by the placenta, stimulates the corpus luteum to continue its production of estrogen and progesterone. Eventually, as the placenta matures, it takes on the role of hormone production, which includes progesterone and estrogen, becoming the primary endocrine organ supporting the pregnancy. The shift to placental hormone synthesis typically occurs around the 12th week of pregnancy.