Final answer:
Most of the estrogen produced during pregnancy comes from the placenta, especially after the first trimester when it takes over the hormone production role from the corpus luteum.
Step-by-step explanation:
During pregnancy, most of the estrogen is produced by the placenta. Initially, from weeks 7 to 12 after the last menstrual period (LMP), the corpus luteum is responsible for the production of pregnancy hormones. However, as the pregnancy advances, particularly from weeks 12 to 17, the placenta becomes the primary endocrine organ, overtaking the role of the corpus luteum which degenerates. The placenta is adept at converting weak androgens from both the maternal and fetal adrenal glands to much more potent estrogens. These hormones are vital for maintaining pregnancy, promoting fetal development, and stimulating the mother's tissue growth. By the time of childbirth, estrogen levels will have increased as much as 30-fold since the beginning of pregnancy.