The syncytiotrophoblast is responsible for producing a hormone called human chorionic gonadotropin (hCG). This hormone signals other parts of the female reproductive system that fertilization and implantation have occurred, so the uterine lining should continue to grow and develop (rather than being shed as menstruation). By the end of the second week of development, sufficient quantities of hCG are produced to be detected in a woman’s urine. The presence of hCG in urine indicates a woman is pregnant, and thus hCG is the basis for modern-day pregnancy tests. For the first 3 months of pregnancy, hCG levels remain high, but after that they decline. By this time, hCG is no longer needed because the placenta is producing its own hormones to maintain the pregnancy.
source :human anatomy
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