Normal growth, sexual dimorphism, and lateral asymmetries of the fetal posterior fossa during the late second and third trimesters

BACKGROUND AND PURPOSE It is important to obtain the accurate volumetric characterization of fetal posterior fossa growth at MRI. This study aims to analyze the normal growth, sexual dimorphism, and lateral asymmetries of the fetal posterior fossa during the late second and third trimesters. MATERIALS AND METHODS 100 fetuses of 23–40 weeks gestational age (GA) with normal brain development were retrospectively included in this study. The bilateral posterior fossa and cerebellar hemisphere, brainstem (including midbrain, pons, and medulla oblongata) were manually segmented on the in vivo fetal MRIs. The normal growth, sexual dimorphism, and lateral asymmetries were analyzed after their volumes were obtained. RESULTS The posterior fossa volume (PFV) linearly increased with GA. The right posterior fossa volume (RPFV) was significantly larger than that of the left. The left posterior fossa volume (LPFV) increased slightly faster than that of the right. The cerebellar volume (CV) increased in the form of power functions with GA. The right cerebellar volume (RCV) was significantly larger than the left cerebellar volume (LCV). The bilateral CVs had a similar growth rate, which apparently accelerated and faster than that of the PFV after 35 weeks GA. The brainstem volume (BSV) linearly increased with GA. The pontine volume (PV) increased the fastest, followed by the midbrain volume (MV), and the medullary volume (MEV) increased the slowest. The relationship between volume of the cerebrospinal fluid (CFV) in posterior fossa and GA was described adequately by a second-order polynomial curve, which increased before 30 weeks GA, while gradually decreased after 35 weeks GA. There were no sexual dimorphisms found in all the measurements. CONCLUSIONS Development of the fetal posterior fossa follows a specific spatiotemporal regularity, and the volumetric measurements demonstrate structure-specific and GA-related developmental trajectories, with different growth rates and lateral asymmetries. The present results are valuable in assessment of normal fetal posterior fossa development in the uterus.