During pregnancy, different protein (hCG and hPL) and steroid (progesterone, oestradiol, oestrone, oestriol) hormones are secreted. hCG, measured by RIA in maternal serum, is a first-trimester trophoblastic function test. The serum levels of hPL reveal the placental function during the two last trimesters of gestation. The main steroid hormones, produced by the foeto-placental unit, are progesterone, oestradiol and oestrone, which control placental function, and oestriol, a good index of foetal vitality.