Progesterone in blood. III. Progesterone in the peripheral blood of pregnant women

The content of progesterone was determined in the peripheral venous blood of pregnant women from the 11th week after the last menstrual period up to the day of birth. There was a steady increase in the blood level from the 11th to the 35th week, but thereafter the level rose more rapidly, and remained high during labour itself. The significance of this rise after the 35th week is not fully understood. It may reflect a decrease in the rate of progesterone metabolism which occurs at this time. The blood level in three cases of twin pregnancy tended to be higher than the corresponding levels for single pregnancies. After removal of the placenta during elective caesarean sections, it was found that the blood progesterone level rapidly fell below the limits of sensitivity of the method. It has been estimated that the half-life of progesterone in the circulating blood must be 5 min or less.