Placental blood flow and the risk of preterm delivery

Abstract

The goal of this analysis was to estimate the influence of variation in uterine artery and umbilical artery resistance indices (RIs) measured across gestation on variation in the risk of preterm delivery (PTD). Analyses were carried out on data collected in a longitudinal study of 523 gravidas. Uterine and umbilical artery RIs were measured on three occasions during pregnancy (16-20 weeks gestation; 21-29 weeks gestation; and 30-36 weeks gestation). Data were analyzed using the Cox proportional hazards regression model. The primary outcome variable was birth prior to 37 weeks gestation. We found that for mothers who delivered preterm the mean uterine artery RI was consistently larger across all gestational ages, while the mean umbilical artery RI decreased significantly more slowly across gestation than for their term counterparts. In analyses pooled by type of delivery, we found that the hazard ratio (HR) for PTD was statistically significant for either uterine artery RI (HR=2.26, 95% CI: 1.65, 3.11) or umbilical artery RI (HR=3.47, 95% CI: 2.43, 4.95) after adjusting for statistically significant covariates. In stratified analyses, the hazard ratio for PTD was also positively associated with an increased uterine or umbilical artery RI in both spontaneous and indicated deliveries. Our data suggest that pregnancies with either a higher uterine or umbilical artery RI across gestation are more likely to be affected by PTD suggesting that disordered placentation resulting in compromised placental blood flow may be an important pathway to PTD.

FIGURE 1

The gestational age dependence of a.) the uterine artery RI; and b.) the umbilical artery RI. Linear regression lines are shown for term (black) and preterm (red) deliveries. For the uterine artery RI, there is no significant difference in the regression coefficients for term and preterm deliveries; there is a 0.21 unit difference in the intercept of the regression lines (P<0.001). For the umbilical artery RI, the regression coefficients for term (β = -0.00147; 95% CI: -0.00151, - 0.00143) and preterm (β = -0.00103; 95% CI: -0.00084, -0.00123) deliveries are significantly different (p<0.01).

FIGURE 1

The gestational age dependence of a.) the uterine artery RI; and b.) the umbilical artery RI. Linear regression lines are shown for term (black) and preterm (red) deliveries. For the uterine artery RI, there is no significant difference in the regression coefficients for term and preterm deliveries; there is a 0.21 unit difference in the intercept of the regression lines (P<0.001). For the umbilical artery RI, the regression coefficients for term (β = -0.00147; 95% CI: -0.00151, - 0.00143) and preterm (β = -0.00103; 95% CI: -0.00084, -0.00123) deliveries are significantly different (p<0.01).