Why hormone levels increase many folds during pregnancy?

During pregnancy, the placenta acts as an endocrine gland, and produces the human chorionic gonadotropins, human placental lactogen, oestrogen, progesterone and relaxin (later stages of pregnancy). Also, hormones like oestrogen, progestogen, cortisol, prolactin, etc., are increased several folds in the maternal blood. These hormones support foetal growth and help in the maintenance of pregnancy.