The objective was to have a quantitative description of the normal position of the fetal midbrain in the first trimester, through defining the reference ranges for the mesencephalon to the occipital bone distance, in the axial plane. This was a prospective study that included normal fetuses screened between 11 and 13 weeks of gestation. The distance was measured between the posterior limit of the mesencephalon to the occipital bone in the same axial view as the one required for the biparietal diameter (BPD) assessment, at this gestational age (GA). The reference ranges using quantile regression, according to the crown-rump length (CRL), BPD, and GA were fitted. Data analysis included 428 ultrasound measurements. A good, linear correlation was observed between mesencephalon to occiput (MO) distance and CRL, BPD, or GA. It increased linearly with advancing gestation (log10MO = -0.1834 + 0.0092 x CRL, R2=0.48, P<0.0001) and was independent of maternal demographic characteristics and intracranial translucency (IT). In our study, the 1st percentile of the normal MO distance varies from 1.31 mm at a CRL of 45 mm to 2.08 mm at a CRL of 84 mm. The intraclass correlation coefficient (ICC) was 0.89 for intraobserver variability. A significant increase in the MO distance was found in the patients who did not receive folic acid in the first trimester of pregnancy [1.056 vs. 1.008 multiple of median (MoM), P=0.014]. A simple measurement is described between the midbrain and the occipital bone, obtained in the same axial view. It increases linearly with advancing gestation. Integration of this measurement into the routine ultrasound screening in association with the 'crash sign' and recognizing the lower extreme values could lead to an early diagnosis of open spina bifida (OSB).
Keywords: first-trimester ultrasound; mesencephalon; midbrain; prenatal diagnosis; spina bifida.
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