Final answer:
The placenta secretes estrogen and progesterone to sustain pregnancy after initially being secreted by the corpus luteum. Progesterone supports embryo growth, while estrogen stimulates tissue growth in the mother and fetus.
Step-by-step explanation:
The organ that secretes enough estrogen and progesterone to sustain pregnancy is the placenta. Initially, these hormones are provided by the corpus luteum until around weeks 12-17 of pregnancy, after which the placenta takes over as the primary endocrine organ. Progesterone is crucial for maintaining the pregnancy as it prevents new ovarian follicles from developing and suppresses uterine contractility, ensuring the embryo can grow undisturbed. Estrogen supports fetal viability and stimulates tissue growth in both mother and fetus. Additionally, the hormone hCG (human chorionic gonadotropin) produced by the outer layer of the blastocyst helps preserve the corpus luteum and its ability to secrete progesterone during early pregnancy.