title: The BS/BSOB Ratio in Aneuploidy Fetuses at 11-13 Weeks Gestation.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Obstetrics and Gynecology, Taipei City Hospital, Women and Children Campus, Taipei, Taiwan ROC.
authors.forename: Ching Hua
authors.initials: CH
authors.lastname: Hsiao
authors.forename: Po Jen
authors.initials: PJ
authors.lastname: Cheng
authors.forename: S W Steven
authors.initials: SW
authors.lastname: Shaw
authors.forename: Yin Jiun
authors.initials: YJ
authors.lastname: Tseng
authors.forename: Ran Chou
authors.initials: RC
authors.lastname: Chen
authors.forename: Woei-Chyn
authors.initials: WC
authors.lastname: Chu
mesh_terms: D000328:Adult; D000782:Aneuploidy; D001933:Brain Stem / Q000000981:diagnostic imaging*; D025063:Chromosome Disorders / Q000000981:diagnostic imaging*; D018568:Crown-Rump Length; D005260:Female; D005865:Gestational Age; D006801:Humans; D009777:Occipital Bone / Q000000981:diagnostic imaging*; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D016216:Ultrasonography, Prenatal / Q000379:methods*