In the first weeks of pregnancy there is a significant increase of vasodilatator prostaglandins in maternal blood. This increase could be in a cause-effect relation with the increase of progesterone, BHCG and HPRL typical of the first phase of pregnancy. Blood samples of 12 normotensive women reveal that there is not a correlation between placental hormons, HPRL and the increase of prostaglandins, but these hormones seem to offer an important control on other more complex biochemical mechanisms that cause the increase of vasodilator prostaglandins.