Developmental characteristics of blood flow in the middle cerebral artery in the human fetus in utero, assessed using the linear-array pulsed Doppler method.

To evaluate the physiological profile of human cerebral circulation, chronological sequences of flow velocity waveforms in the middle cerebral artery were studied using a pulsed Doppler method. Included were a total of 92 normal fetuses between 27 and 41 weeks of gestation, among which waveforms could be recorded in 81 cases (88.0%). Raw data of the waveforms were analysed by means of two indices: resistance index (RI) and pulsatility index (PI). Both RIs and PIs were calculated at 3-week intervals and were compared at two consecutive periods. Both RI and PI remained unchanged between 27 and 35 weeks of gestation. There were significant decreases in both indices between 33-35 and 36-38 weeks (P less than 0.01), and also between 36-38 and 39-41 weeks (P less than 0.05). These findings indicate that cerebral vascular resistance starts to decrease at the critical period of 36-38 weeks, after which there is a continuing decrease to term. The findings obtained using our method of assessment of fetal middle cerebral artery circulation are discussed.