The placenta and especially the syncytiotrophoblast can be seen as a large endocrine gland.
Before implantation hormone production is ensured through ovarian and hypophysial hormones.
At the beginning of the pregnancy the synthesis of estrogen and progesterone is ensured by the corpus luteum graviditatis that is maintained by the human chorion-gonadotropin (HCG), a product of the trophoblast. The activity of the corpus luteum decreases progressively with the beginning of the 8th week in order to be entirely replaced by the placenta at the end of the 1st trimester.
During the pregnancy the hormone concentration in the maternal blood is regulated by the cooperation of the placental, hypophysial and fetal suprarenal hormones as well as hormones from the gonads.