Objective: The study aimed to describe reference values for structures of the posterior fossa in fetuses with a crown-rump length (CRL) between 45 and 84 mm. Materials and Methods: This was a prospective, cross-sectional study including 216 normal appearing fetuses. In transvaginal acquired 3-dimensional volume blocks, the longest diameter of the vermis (VE), posterior membranous area (PMA), medulla-oblongata-pons angle (MOPA), diameters of the medulla oblongata (MO) and pons (PO), and the area of Blake’s pouch (BP) were measured. Polynomial or linear regression analysis were performed to calculate the mean, 5th and 95th centile according to CRL. In 20 fetuses, intra- and interobserver repeatability were calculated. Results: There is a curvilinear correlation between CRL and PO (PO [mean] = 1.3893 + 0.004356 × CRL + 0.000002610 × CRL3; SD = 1.6818 – 0.03765 × CRL + 0.000003831 × CRL3; R2 = 0.489); CRL and MO (MO [mean] = 1.5959–0.001905 × CRL + 0.000003362*CRL3; SD = –0.1417 + 0.005404 × CRL + 0.0000004988 × CRL3; R2 = 0.525); CRL and VE (VE [mean] = –0.3640 + 0.04302 × CRL+ 0.000001486 × CRL3; SD = 0.5854 – 0.004812 × CRL + 0.0000005896 × CRL3; R2 = 0.643); CRL and PMA (PMA [mean] = 0.6901 + 0.04307 × CRL – 0.0000008459 × CRL3; SD = –0.4232 + 0.02026 × CRL – 0.000001320 × CRL3; R2 = 0.272); CRL and BP (mm2; BP [mean] –12.2067 + 0.3334 × CRL – 0.00001262 × CRL3; SD = –1.6431 + 0.06380 × CRL+ 0.0000003257 × CRL3; R2 = 0.289). The relation between CRL and MOPA (°) is best described by a linear regression (MOPA [mean] = 79.6332 + 0.6122 × CRL; SD = 4.8453 + 0.07333 × CRL; R2 = 0.318). Conclusion: We provide reference values for anatomical structures of the posterior fossa of fetuses between 45 and 84 mm CRL. The established reference values might ease the diagnosis of fetal malformations in early pregnancy.

Keywords:
1 st Trimester screening, Fetal fossa posterior, Reference ranges, Hypoplasia/Aplastia of the vermis, Transvaginal 3D ultrasound
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2018
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