The act of giving birth to young at the end of the gestation period. In most mammals it is triggered by the secretion of ACTH by the fetal pituitary gland, which causes increased secretion of corticosteroid hormones by the fetal adrenal glands. Cortisol produced by the fetus stimulates the activity of a placental enzyme that increases the level of oestradiol in the bloodstream, which in turn enhances the synthesis of prostaglandins by the uterus, leading to the muscular contractions of labour. Other key hormones include oxytocin and relaxin. In women this broad picture is made more complex by multiple paracrine, autocrine, and endocrine signals and various maternal and fetal control mechanisms.