The following peptide hormones are secreted by the placenta:  Human chorionic gonadotropin (hCG) is required for implantation and maintenance of the pregnancy. Its synthesis begins around day 6, even before the syncytiotrophoblast is formed. HCG exhibits extensive (about 85%) sequence homology to LH, which is required for ovulation and maintenance of the corpus luteum during the menstrual cycle. Similar to the work of LH during the menstrual cycle, hCG maintains the corpus luteum during early pregnancy. HCG also possesses marked homology to pituitary thyroid-stimulating hormone (TSH), which may account for hyperthyroidism in pregnancy by stimulating the maternal thyroid gland to increase secretion of tetraiodothyronine (T4). Measurement of hCG is used to detect early pregnancy and assess pregnancy viability. Two other clinical conditions that increase the blood levels of hCG include trophoblastic diseases and ectopic pregnancies.
 Human chorionic somatomammotropin (hCS), also known as human placental lactogen (hPL), is closely related to human growth hormone. Synthesized in the syncytiotrophoblast, it promotes general growth, regulates glucose metabolism, and stimulates mammary duct proliferation in the maternal breast. The effects of hCS on maternal metabolism are significant, but the role of this hormone in fetal development remains unknown.
 IGF-I and IGF-II are produced by and stimulate proliferation and differentiation of the cytotrophoblast.
 Endothelial growth factor (EGF) exhibits an age-dependent dual action on the early placenta. In the 4to 5-week-old placenta, EGF is synthesized by the cytotrophoblast and stimulates proliferation of the trophoblast. In the 6to 12-week-old placenta, synthesis of EGF is shifted to the syncytiotrophoblast; it then stimulates and maintains the function of the differentiated trophoblast.
 Relaxin is synthesized by decidual cells and is involved in the “softening” of the cervix and the pelvic ligaments in preparation for parturition.
 Leptin is synthesized by the syncytiotrophoblast, particularly during the last month of gestation. Leptin appears to  FOLDER 23.4 Clinical Correlation: Fate of the Mature Placenta at Birth