The increase of blood flow in the fetal middle cerebral artery correlates with the onset of labor at term.

The objective of this cohort study is to evaluate the relationship between blood flow resistance in the fetal middle cerebral artery (MCA) and the beginning of spontaneous labor. A group of consecutive women (n = 664) with a singleton pregnancy was submitted to Doppler examination of fetal MCA between 24 and 41 weeks' gestation (median, 35 weeks). The study population was later divided into 2 groups according to the MCA pulsatility index (PI), expressed in multiples of the median (MoM) for each gestation week. The median time elapsed between the Doppler examination and the onset of spontaneous labor (which eventually occurred in 302 women, of whom 291 were at term) was significantly shorter in the group with MCA PI <0.74 MoM (5.5 days; interquartile interval, 2-10 days) than in the group with MCA PI >or=0.74 MoM (22.5 days; interquartile interval, 5-37.5 days; P < .001). Survival analysis and Cox regression confirmed that the MCA PI was independently associated with the number of days elapsed from Doppler to spontaneous labor (P < .001; Exp[B], 2.77; 95% confidence interval, 1.95-3.90) after correction for birth weight and umbilical artery PI. The present data suggest that, at term of pregnancy, a gestation-independent decrease of fetal cerebral vascular resistance precedes the onset of spontaneous labor.