A prospective cohort study was conducted to determine whether preterm labor is associated with changes in the impedance to blood flow of the fetal middle cerebral artery. Doppler velocimetry studies were performed in 194 consecutive patients with preterm labor and intact membranes. Pulsatility indices of the middle cerebral artery and umbilical artery were determined on admission. Results were expressed as ratio of the observed pulsatility index to mean value for gestational age expressed as delta MCA PI and delta UA PI, respectively. The prevalence of preterm delivery (< 37 weeks) and delivery within 24 hours of admission was 55.2% (107/194) and 15.5% (30/194), respectively. Patients with an examination-to-delivery interval within 24 hours had significantly lower mean delta MCA PI than that of patients delivered at > or = 37 weeks and > or = 4 weeks after the examination (P < 0.01). Fetuses with a delta MCA PI at or below 0.88 had a relative risk of 2 (95% confidence interval, 1.32 to 2.9) who were delivered within 24 hours compared to controls (sensitivity 76.7%, specificity 62.8%, positive predictive value 26.7%, negative predictive value 93.5%). Stepwise logistic regression analysis indicated that the relationship between delta MCA PI and examination-to-delivery interval remained statistically significant after correcting for cervical dilatation (P < 0.001). Our data indicate that preterm parturition is associated with a decrease in the impedance to flow in the fetal cerebral circulation.
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