Immune rejection in fetus is prevented by

Chorionic gonadotropin (human chorionic gonadotropin (HCG) (hCG) is a glycopeptide hormone that is produced by cells of the fetal placenta and maintains the function of the corpus luteum during the first few weeks of pregnancy. It is thought to promote steroidogenesis in the fetoplacental unit and tostimulate fetal testicular secretion of testosterone. It can be detected by immunoassay in the maternal urine within days after feilization; this provides the basis for the most commonly used pregnancy test. Due to its highly negative charge, hCG may repel the immune cells of the mother, protecting the fetus during the first trimester. It has also been hypothesized that hCG may be a placental link for the development of local maternal immunotoleranceRef: Ganong&;s review of medical physiology;24th edition; page no:-414