The development of the placenta begins when the blastocyst evokes the decidual reaction in the maternal endometrium5a and transforms this into an exceedingly well perfused source of nutrition - the basal plate. The neighboring glands of the endometrium develop and the uterine wall gets to be turgid locally. If an implantation has taken place the trophoblast cells release a multiplicity of hormones (HCG = human chorionic gonadotrophine, HCS = human chorionic somatomammotrophine, and also HPL = human placental lactogen) that are necessary to coordinate the nourishment of the endometrium and to maintain the corpus luteum which, over roughly 12 weeks of the embryonic development, continues to secrete sexual steroids. Later, the placenta itself starts to secrete large amounts of progesterone and thereby takes over the glandular function of the corpus luteum.
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