Twenty normotensive women at term in early labor at B. C. Women's Hospital were approached to participate in this study. The study was approved by the University of British Columbia Ethical Review Board'. All patients enrolled in this study signed a written informed consent and received a copy of the consent form before participation in this study. 

Inclusion criteria for the study were (1) normotensive; (2) gestational age 40 weeks +(-)2 weeks; (3)no history of antepartum problems; (4) in active first stage of labor, with cervical dilatation of 4 to 6 cm and strong uterine contractions every 3 to 4 minutes lasting at least 90 seconds on palpation.

Middle cerebral artery blood flow velocity was measured using bidirectional pulsed-wave Doppler ultrasonography with a 2 MHz probe and a Fast Fourier Transform real-time spectrum analyzer. The maternal middle cerebral artery blood flow velocity was assessed by placing the ultrasound transducer against the temporal window (above the zygomatic arch 1 to 3 cm in front of the temple) and by adjusting the depth of the ultrasound beam. The depth at which good signals were obtained was 55 mm corresponding to the initial portion of the maternal cerebral artery. The right and left middle cerebral arteries were evaluated, and the average of the readings from both sides was used for analysis. All patients were scanned by one investigator, with a previously documented intraobserver coefficient of variation of 2.5% in the assessment of middle cerebral artery blood flow velocity. We measured velocity data in centimeters per second. We used the velocity wave during four successive cardiac cуcles and assessed maximum systolic, minimum diastolic, and mean velocity. The pulsatility index was defined as the difference between the maximum systolic velocity and the minimum diastolic velocity divided by the mean velocity.

All patients were assessed (1) at the trough of a contraction, (2) at the peak of a contraction, and (3) during the second stage of labor when actively pushing with contractions. Mean group values for cerebral blood flow velocity were calculated. Simultaneous assessment of blood pressure and maternal heart rate was done using a Dinamap monitor ( Gainesville, Fla.). Analysis revealed thạt the data were normally distributed and subsequently statistical analysis was performed by use of parametric tests. Mean values between the groups for continuously distributed variables such as cerebral blood flow velocity and blood pressure were compared by use of analysis of variance. Tukey test with Tukey-kramer adjustment was used when necessary to assess the significance. Statistical significance was set at p<0.05.

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