目的　利用MRI测量孕中晚期正常胎儿小脑横径及体积，研究其随孕周的发育规律，并探讨其在单纯后颅窝池增宽诊断中的临床价值。  方法　本研究前瞻性纳入139例孕周为24~38周的正常胎儿，行胎儿颅脑1.5T MRI检查，在三维后处理工作站（OsiriX软件）上测量小脑横径（TCD）、体积（CV），分析其随孕周变化的规律；另外同期纳入45例经产前超声及磁共振诊断为胎儿单纯后颅窝池增宽作为研究组，将其按孕周划分为两组（I组：28~32周，20例；II组，33~38周，25例），同时按孕周及例数匹配45例正常胎儿作为对照组，定量分析单纯后颅窝池增宽时胎儿小脑发育的情况。   结果　①胎儿小脑横径与孕周呈线性增长，直线方程为TCD=-1.746+0.182GA（R2=0.953，P<0.05），胎儿小脑体积与孕周呈指数增长，曲线方程为CV=0.072exp（0.151GA）（R2=0.957,P<0.05）；②单纯后颅窝池增宽胎儿与正常胎儿相比分析得出：I组，研究组胎儿小脑横径及体积与对照组间对比无统计学差异（P均>0.05）；II组，研究组胎儿小脑横径及体积与对照组对比无统计学差异（P均>0.05）。  结论　孕中晚期正常胎儿小脑横径及体积随孕周增加分别呈线性及指数增长趋势，此结果有助于评估胎儿小脑发育程度；胎儿单纯后颅窝池增宽与小脑横径及体积无明显相关性，这有助于对后颅窝池增宽的临床意义再认识。

Objective　To measure the transverse diameter and volume of normal fetal cerebellum by MRI, study its developmental rule with gestational age, and explore its clinical value in the diagnosis of simple posterior fossa pool broadening.　Methods　This study was a prospective study, involving 139 fetuses with normal brains ranging in age from 24 to 38 gestational weeks, fetal brain was performed 1.5T MRI , then we measured the cerebellum transverse diameter (TCD) and volume (CV) by a three-dimensional post-processing workstation (OsiriX software), and analyszed its developmental rule with gestational age; In addition, 45 cases of prenatal ultrasound and magnetic resonance diagnosis were included in the study. The fetus were divided into two groups according to gestational age (group I: 28~32 weeks, 20 cases; group II, 33~38 weeks, 25 cases), simultaneously according to the gestational age, the number of cases matched 45 normal fetus as a control group, quantitative analysis of fetal cerebellar development when the posterior fossa pool was widened.  Results  ①The fetal cerebellar transverse diameter was linearly positively correlated with gestational age. The linear equation was TCD=-1.746+0.182GA（R2=0.953，P<0.05）, the fetal cerebellar volume was exponential-dependent with gestational age, and the curve equation was CV=0.072exp(0.151GA)（R2=0.957，P<0.05)；②The analysis of the widened fetus in the posterior fossa pool compared with the normal fetus：In group I, there was no significant difference in the diameter and volume of fetal cerebellum between the study group and the control group（both P>0.05）；In group II, there was no significant difference in the transverse diameter and volume of fetal cerebellum between the study group and the control group (both P>0.05).　Conclusions　The transverse diameter and volume of the normal fetal cerebellum increased linearly and exponentially with the gestational age in the second and third trimester of pregnancy. This result is helpful to evaluate the degree of fetal cerebellum development. There is no significant correlation between the fetal posterior fossa pool broadening and the cerebellar transverse diameter and volume. This helps to re-recognize the clinical significance of the posterior fossa pool broadening.