OBJECTIVE The purpose of our study was to evaluate the changes occurring in maximum flow velocity and waveform patterns of the portal and hepatic veins during pregnancy in second and third trimesters. METHODS A total of 204 patients were randomly divided into the study and control groups. The control group was classified as group I; the study group was divided into two groups: group II, second trimester pregnancy (14-26 weeks, n = 91) and group III, third trimester pregnancy (26-40 weeks, n = 63). Vein waveforms were classified as triphasic, biphasic and monophasic. RESULTS Maximum flow velocities of both right hepatic vein and main portal veins were lower in group III compared with group I and group II (p < 0.05). On the basis of hepatic vein and portal vein waveforms, the incidence of biphasic pattern was prominent in group II (p < 0.05) whereas the monophasic pattern was prominent in group III (p < 0.05). CONCLUSIONS The results demonstrate that as gestational age progresses, maximum flow velocities of both right hepatic vein and main portal veins intend to be decreased with an increased incidence of monophasic wave form pattern, and may be accepted as sensitive parameters of indicators of physiological adaptations related to pregnancy.