They form the outermost fetal component of the placenta (also known as 'syntrophoblast') and massively increase the surface area available for nutrient exchange between the mother and the fetus.
Cells from the syncytiotrophoblast secrete progesterone in addition to human chorionic gonadotropin (hCG); the latter prevents degeneration of the corpus luteum. Progesterone serves to maintain the integrity of the uterine lining, and until the syncytiotrophoblast is mature enough to secrete enough progesterone (in the fourth month of embryonic development), it is aided by the corpus luteum graviditatis. [1]
Musicki B, Pepe G, Albrecht E (1997). "Functional differentiation of placental syncytiotrophoblasts during baboon pregnancy: developmental expression of chorionic somatomammotropin messenger ribonucleic acid and protein levels.". J Clin Endocrinol Metab82 (12): 4105–10. doi:10.1210/jc.82.12.4105. PMID 9398722.Article
