Identification of bilateral fornix and hippocampus in fetuses between 18-36 gestational weeks and establishment of nomograms using ultrasonography.

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To establish nomograms for the bilateral fornix-hippocampus complex (FHC) length and hippocampus height (HH) in fetuses between 18 + 0 and 36 + 0weeks of gestation using two-dimensional (2D) sonography and assess potential laterality differences. A prospective study was conducted on 725 singleton pregnancies at the Maternal-Fetal Medicine Department from January to December 2024. Gestational age was confirmed via first-trimester crown-rump length measurements. Exclusion criteria included maternal chronic diseases, multiple pregnancies, fetal anomalies, and growth restriction. Transabdominal 2D sonography captured FHC and HH measurements in the parasagittal plane. Laterality was determined before imaging, and each measurement was repeated twice for reliability. Percentiles (5th, 50th, and 95th) were calculated, and Pearson correlation coefficients assessed relationships with gestational age. The independent samples t-test compared left and right measurements, and intraobserver reliability was evaluated with the intraclass correlation coefficient (ICC). Nomograms with 5th, 50th, and 95th percentiles were established for both sides. The Pearson correlation coefficients for the left and right sides of the FHC length and HH with advancing gestational age were 0.808, 0.808, 0.725, and 0.734, respectively. The correlation coefficients between FHC length and HH for the left and right sides were 0.814 and 0.818, respectively. No significant laterality differences were found (p > 0.05). These nomograms provide reference data for assessing fetal neurodevelopment. They may aid in detecting abnormalities, particularly in corpus callosum agenesis and ventriculomegaly.