Background: Blood flow resistance of the subplacental spiral arteries in second-trimester pregnancies has not been previously reported. A reference range of blood flow resistance of the subplacental spiral arteries at 13-25 gestational weeks was designed in the hope the reference data could provide a basis for Doppler studies of pathologic disorders in second-trimester pregnancies.
Methods: A cross-sectional study was performed in 175 uncomplicated pregnancies at 13-25 gestational weeks. Doppler flow examinations of the subplacental spiral arteries were done.
Results: In the 175 normal pregnancies, both the predicted systolic/diastolic ratios(S/ D) (Y = 1.96258-0.02061x gestational age (GA), adjusted R2 = 0.03773, p = 0.003) and resistance indices (RI) (Y = 0.52131-0.00871 x GA, adjusted R2 = 0.03797, p = 0.003) of the subplacental spiral arteries decreased progressively with advancing gestational age. The predicted S/D value of the subplacental spiral artery decreased from 1.695 at the 13th week's gestation (5th% = 1.221, 95th% = 2.169) to 1.468 at the 24th week's gestation (5th% = 1.057, 95th% = 1.878). The predicted RI value of the subplacental spiral artery also decreased from 0.408 at the 13th week's gestation (5th% = 0.193, 95th% = 0.623) to 0.312 at the 24th week's gestation (5th% = 0.148, 95th% = 0.477).
Conclusions: Normal blood flow resistance of the subplacental spiral arteries at 13-25 gestational weeks decreases progressively with advancing gestational age. The fact suggests that trophoblastic invasion of the spiral arteries occurred continuously throughout normal second-trimester pregnancies.