Spiral artery blood flow in the central and peripheral areas of the placental bed in the second trimester.

OBJECTIVE: To assess impedance to blood flow through spiral arteries in the central and peripheral areas of the placental bed, to determine if in vivo findings are in concordance with histologic observations of cytotrophoblast invasion. METHODS: This study was cross-sectional, using transabdominal color flow and pulsed Doppler imaging in 64 women with normal, singleton pregnancies at 17-20 weeks' gestation. Spiral arteries were visualized by color Doppler in the central and peripheral parts of the placental bed, and blood flow was analyzed using pulsed Doppler to calculate the resistance index (RI) and the pulsatility index (PI). RESULTS: Visualization of flow in the spiral arteries was achieved in 100% of cases in the central area and in 95% in the periphery. Both the RI and PI were significantly lower in the center compared with the periphery (P < .001). There was no significant difference in the peak systolic velocities between the two areas. Regression analysis showed no significant difference in RI or PI at the different weeks of gestation in the central and peripheral areas of the placental bed. CONCLUSION: Impedance to blood flow through spiral arteries in the second trimester is lower in the central area of the placental bed, and is in agreement with previous histologic data. Physiologic change of the spiral arteries is functionally complete around 17 weeks' gestation.