Reference Ranges of Maximum Diameter and Volume of the Fetal Large Intestine During the Second-to Third-Trimester: An MRI Study.

Objective: To investigate normal reference ranges for the maximum diameter and volume of the fetal large intestine across the second and third trimesters, and to analyze their correlations with gestational age (GA).

Method: A single-center, retrospective analysis was conducted on fetal magnetic resonance imaging (MRI) data between 2019 and 2025 at a tertiary hospital in eastern China. We included second-to third-trimester fetuses without prenatal detection of gastrointestinal tract anomalies and with postnatal follow-up (at least 1 week) confirming normal intestinal function, including successful feeding and meconium passage. The maximum diameter and volume of the large intestine were measured, and their correlations with GA and abdominal circumference (AC) were analyzed. Gestational week-specific percentile reference ranges and regression equations for the maximum diameter and volume of the fetal large intestine relative to GA were established.

Results: The large intestine was successfully measured in 387 fetuses. Both the maximum diameter and volume of the fetal large intestine increased with advancing GA and AC. The regression models based on the GA were as follows: large intestinal maximum diameter = 1.0274 × e^(0.0712×GA) (R² = 0.656, p < 0.01); large intestinal volume = 0.0002 × GA^3.501 (R² = 0.756, p < 0.01).

Conclusion: This study provides normal reference ranges for the fetal large intestinal maximum diameter and volume from the second to the third trimester. The findings herein may contribute to improved differentiation between physiological and pathological intestinal changes.