The posterior fossa of normal fetuses was evaluated and compared with those having chromosomal abnormalities at 11-13+6 weeks' gestation in Chinese population. Methods: In 518 normal fetuses referred to first trimester screening, fetal brain stem (BS) and brain stem to occipital bone distance (BSOB) were measured prospectively. The BS and BSOB were also measured on stored images in fetuses with confirmed trisomy 21 (n = 38), Trisomy 18 (n = 26), Trisomy 13 (n = 8), and monosomy X (n = 8). Results: The BS diameter and BSOB distance correlated linearly with fetal crown-rump length (CRL) by regression analysis. The BS to BSOB ratio was below the 5th percentile in 2 (5.26%), 11 (44%), 4 (50%) and 4 (50%) fetuses with trisomy 21, trisomy 18, trisomy 13 and monosomy X, respectively. Thus, both BS and BS/BSOB ratio were significantly lower in trisomy 18, trisomy 13 and monosomy X fetuses when compared to the reference range but not in fetuses with Trisomy 21. Conclusion: In ultrasound scans performed at the 11-13+6 gestation weeks, fetuses with trisomy 18, 13, and monosomy X had lower BS/BSOB ratios. But trisomy 21 fetuses did not show significant differences in posterior fossa compared to the normal population.