Objective To assess reference ranges of blood flow parameterratios of the normal second-trimester fetal middle cerebral artery.Methods Blood flow parameters of middle cerebral artery,head circumference and fetal weight were assessed in 215 normal fetus at 16 to 31 weeks gestation by two-dimension imaging,color Doppler power imaging and pulsed Doppler.Results During 16 to 31 weeks gestation,unit weight maximum and mean blood flow velocities per minute of fetal middle cerebral artery were decreased with gestation age.Unit head circumference maximum and mean blood flow velocities per minute of fetal middle cerebral artery were changed little during the normal second-trimester.Conclusion This study offers a theoretic basis to the early prediction of fetal hyperdynamic circulation with Doppler ultrasound.%目的确定正常中孕胎儿大脑中动脉血流参数比值的正常值范围。方法应用二维超声、彩色多普勒能量图及脉冲多普勒技术对215例16～31孕周正常胎儿头围、体重及大脑中动脉血流参数进行检测。结果正常中孕胎儿每分钟单位体重大脑中动脉的最大速度及平均速度随孕周的增加而降低；每分钟单位头围大脑中动脉的最大速度及平均速度在中孕期间变化小。结论本研究为多普勒超声早期预测胎儿高动力循环状态提供了理论依据。
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