Haemostasis in pregnancy plays a critical role in childbirth. Research purposes: 1/ To study the changes in the haemostasis during the third trimester of pregnancy at Bach Mai Hospital from February to August 2010; 2/ To investigate changes some coagulation parameters in pregnancies with preeclampsia. Study method: controlled cross-sectional survey. Results and conclusions: 1. Haemostasis during the third trimester of pregnancy: the platelet count decreases, increases in the majority of clotting factors, increases in fibrinolytic activity; 2. Haemostasis in pregnancies with preeclampsia: reduction in platelet count and activation of the extrinsic coagulation pathway is more severe in comparison with pregnant woman with no preeclampsia.