Reference Ranges for Fetal Parieto‐Occipital Sulcus Depth and Angle Using the Transcranial High Axial Plane: A Cross‐Sectional Study at 20–30 ⁺⁶  Weeks of Gestation

ABSTRACT Objective To establish gestational age (GA)‐specific reference ranges for fetal parieto‐occipital sulcus (POS) depth and angle using the transcranial high axial plane at 20–30 +6 weeks of gestation. Methods This cross‐sectional study enrolled 357 normal singleton fetuses. Five cases were excluded owing to suboptimal image quality caused by maternal obesity, unfavorable fetal presentation, anterior placenta and oligohydramnios. Finally, 352 fetuses were included for analysis, yielding a POS visualization rate of 98.6% (352/357). Gestational age (GA) was established based on first‐trimester crown‐rump length (CRL) measurements. The morphology of the POS was assessed, and its depth and angle were measured to establish relevant reference ranges. Intra‐ and inter‐observer reproducibility was evaluated. Results POS depth showed a linear increase with advancing gestational age (GA), whereas POS angle presented a linear decrease. The regression equations were: POS depth (mm) = −16.41 + 0.85 × GA ( R 2 = 0.853, p < 0.01) and POS angle (°) = 389.06–11.82 × GA ( R 2 = 0.855, p < 0.01). Reference ranges for POS depth and angle were established. Excellent reproducibility was observed, with all intraclass correlation coefficients (ICCs) > 0.850, and more than 95% of data points lying within the 95% limits of agreement (LoA) on Bland–Altman plots. Conclusion Quantitative measurements of POS depth and angle on the transcranial high axial plane offer a simple, feasible, and reproducible complementary method to evaluate fetal cerebral cortical development during 20–30 +6 weeks of gestation.