Reduction in protein S activity during normal pregnancy.

We investigated the serial changes in blood protein S (PS) and related proteins in 11 normal pregnant women. The PS activity decreased significantly in the third trimester and reached minimum levels (23.3%) one hour after delivery. Although the PS activity was reduced markedly below the normal limits, all the women delivered safely. The mechanisms that cause the reduction in PS activity and the clinically dangerous conditions involving PS activity during pregnancy warrant further investigation.