Objectives: Assessment of uterine artery blood flow in women with congenital thrombophilia and antiphospholipid syndrome (APS) in the first half of pregnancy Material and methods: Uterine arteries blood flow was assessed in a Doppler examination in 20 women with thrombophilia (15 with congenital thrombophilia, 5 with APS) at 12 and 20 weeks gestation at the Division of Reproduction, Poznan University of Medical Sciences, between 2000 and 2012 The control group consisted of 20 multiparous pregnant women with no history of pregnancy complications. All patients with thrombophilia received enoxaparin or enoxaparin and aspirin before enrollment into the study. Patients from the control group did not receive any antithrombotic prophylaxis. The mean Pulsatility Index (PI) of both uterine arteries and the presence or the absence of the “notch” was assessed, both at 12 and 20 weeks gestation in each patient from the study and from the control groups. Results: Mean PI values in the uterine arteries at 12 weeks in patients with thrombophilia and in controls were 1.82 (1.00-3.13) and 1.52 (1.30-1.88), respectively (p=0.08). Mean PI value in the uterine arteries was 1.27 (0.61-2.48) in women with thrombophilia at 20 weeks, which turned out to be significantly higher (p=0.026) than in the control group 1.07 (0.8-1.24). The bilateral “notch” was found at 12 weeks gestation in 40% of patients with thrombophilia vs. 0% in the control group (p=0.03). There was no significant difference between the groups in this parameter at 20 weeks. Conclusions: 1. An increased impedance of flow was found in the uterine arteries in patients with thrombophilia at 12 and 20 weeks gestation in spite of antithrombotic prophylaxis. 2. Thrombotic episodes in patients with thrombophilia cannot be explained solely by the presence of placental thrombosis