During the second and third trimesters of human gestation, rapid neurodevelopment is underpinned by fundamental processes including neuronal migration, cellular organization, cortical layering, and myelination. In this time, white matter growth and maturation lay the foundation for an efficient network of structural connections. Detailed knowledge about this developmental trajectory in the healthy human fetal brain is limited, in part, due to the inherent challenges of acquiring high-quality MRI data from this population. Here, we use state-of-the-art high-resolution multishell motion-corrected diffusion-weighted MRI (dMRI), collected as part of the developing Human Connectome Project (dHCP), to characterize the in utero maturation of white matter microstructure in 113 fetuses aged 22 to 37 wk gestation. We define five major white matter bundles and characterize their microstructural features using both traditional diffusion tensor and multishell multitissue models. We found unique maturational trends in thalamocortical fibers compared with association tracts and identified different maturational trends within specific sections of the corpus callosum. While linear maturational increases in fractional anisotropy were seen in the splenium of the corpus callosum, complex nonlinear trends were seen in the majority of other white matter tracts, with an initial decrease in fractional anisotropy in early gestation followed by a later increase. The latter is of particular interest as it differs markedly from the trends previously described in ex utero preterm infants, suggesting that this normative fetal data can provide significant insights into the abnormalities in connectivity which underlie the neurodevelopmental impairments associated with preterm birth.

在人类妊娠的第二和第三孕期，快速的神经发育是由包括神经元迁移、细胞组织、皮质分层和髓鞘形成在内的基本过程支撑的。此时，白质的生长和成熟为有效的结构连接网络奠定了基础。关于健康人类胎儿大脑中这种发育轨迹的详细知识是有限的，部分原因是从该人群中获取高质量 MRI 数据的固有挑战。在这里，我们使用最先进的高分辨率多壳运动校正扩散加权 MRI (dMRI)，作为开发中的人类连接组计划 (dHCP) 的一部分收集，以表征子宫内白质微结构的成熟。 113 个胎儿年龄在 22 至 37 周妊娠。我们定义了五个主要的白质束，并使用传统的扩散张量和多壳多组织模型来描述它们的微观结构特征。与关联束相比，我们发现了丘脑皮质纤维的独特成熟趋势，并确定了胼胝体特定部分的不同成熟趋势。虽然在胼胝体压部观察到分数各向异性的线性成熟增加，但在大多数其他白质束中观察到复杂的非线性趋势，在妊娠早期分数各向异性最初降低，随后增加。后者特别令人感兴趣，因为它与之前在宫外早产儿中描述的趋势明显不同，