The concept of the foeto-placental unit as an integrated endocrine organ has been defined recently by many in vivo studies at the 17th- 20th week of gestation. A functioning foeto-placental unit is necessary for most of the increased oestrogen production of pregnancy and for the provision of glucocorticoids and aldosterone to the foetus. Neither the foetus nor the placenta alone have the necessary enzyme systems for the synthesis of these groups of steroids. However, when the foetus and placenta function as a unit, all of the enzyme systems are present for the synthesis of these steroids from...