Summary
Holtzman rats, 16 to 19 days pregnant, exhibited in vitro hypercoagulability with
a 27 % reduction in the plasma clotting time (PCT) (siliconized and plastic) as compared
to non-pregnant controls. This hypercoagulability was entirely due to an increased
activity of platelet factor 3, since a comparable reduction in the clotting time was
obtained when the washed platelets from these animals were resuspended in a standard
platelet-poor plasma. In contrast to this, when the platelets in the original plasma
were replaced by a phospholipid suspension, the clotting time in plastic but not in
glass, was prolonged in pregnant rats, suggesting a decreased activity of the contact
system. In addition, the pregnant rats exhibited a marked increased susceptibility
of platelets to thrombin and to ADP-induced aggregation.
In 6 pregnant women and in 9 women taking contraceptives, a highly significant in
vitro hypercoagulability was also present, since the PCT was decreased 26% and 22%,
respectively, as compared to the 14 controls. This hypercoagulability appeared to
be entirely due to the increased activity of platelet factor 3 as in rat. However,
the clear-cut increased susceptibility of platelets to thrombin and ADP-induced aggregation
noted in the pregnant rats was not obtained in either the pregnant women or in the
women taking contraceptives.
