Maternal cerebral blood flow changes in pregnancy

OBJECTIVE: This study was undertaken to determine blood flow changes in the large cerebral arteries during normal pregnancy. STUDY DESIGN: Ten healthy pregnant volunteers underwent velocity-encoded phase contrast magnetic resonance imaging at 4 time intervals: 14 to 16, 28 to 32, and 36 to 38 weeks' gestation, and at 6 to 8 weeks' postpartum. Analysis consisted of serial paired Student t tests, with P < .05 considered significant. RESULTS: By using postpartum values for comparison, cerebral blood flow decreased by 14 to 16 weeks in the middle cerebral artery (P < .001), but was not significantly changed in the posterior cerebral artery. Significant decreases occurred in both the middle (P < .0001) and posterior (P = .002) cerebral arteries in late pregnancy. CONCLUSION: An approximately 20% reduction in large artery cerebral blood flow occurs during normal pregnancy, secondary to changes in velocity, whereas the area of these vessels remains unchanged. These findings may represent generalized vasodilatation of downstream resistance arterioles, assuming constant blood flow at the tissue level.