Is that bowel normal? Nomograms for fetal colon and rectum measurements by MRI from 20-36 weeks' gestation

MRI is being increasingly used as a supplemental tool to ultrasound for prenatal diagnosis of fetal anomalies. Evidence suggests fetal MRI may aid in diagnosis and management of gastrointestinal abnormalities, though it is not widely used for this indication at this time. There is a lack of well-established nomograms of fetal colonic measurements by gestational age (GA) with MRI.

We aim to establish a nomogram of normal colonic and rectal size throughout the second and third trimesters by fetal MRI, starting at a younger GA than previously documented.

A retrospective study was performed. Fetal MRI databases spanning October 2010 to June 2021 were searched for examinations performed from 19 weeks GA to term. Cases were excluded for poor image quality or if a gastrointestinal abnormality was noted. Coronal and sagittal T1-weighted images were used to measure the ascending, transverse, descending, and sigmoid colon; rectosigmoid colon (at the level of the iliac crest); and rectum (at the distal third in the coronal views and at the base of the bladder and largest dimension in the sagittal views, at the orthogonal plane of the colon). Reproducible measurement areas for fetal bowel were established between two readers, and two-way mixed single-measure intra-class correlation coefficients (ICCs) were developed to assess inter-rater reliability. Growth curves were developed using generalized additive models. The main outcomes were the measurements at each section of the bowel and the predictor was GA. The curves of the fitted models were plotted for the estimated 10th, 25th, 50th, 75th, and 90th percentiles.

A total of 315 patients had at least one measurable bowel segment. If 100 or more measurements were obtained at a bowel segment, a nomogram was created. Due to limited available data prior to 20 weeks and after 36 weeks GA, bowel nomograms were developed for 20-36 weeks GA. The three bowel segments with the most included patients and greatest number of measurements obtained were the rectosigmoid colon at the iliac crest, distal one-third of the rectum, and the sigmoid colon (n = 251, 234, 178, respectively) with excellent reader agreement (ICC = 0.96, 0.90, 0.94, respectively). The range of bowel diameter increased with advancing GA at the 10th percentile through the 90th percentile in a nearly linear distribution at the sigmoid colon, rectum at the base of the bladder, and distal rectum and in a non-linear distribution at the rectum at the level of the iliac crest and descending colon.

Our study provides normative data for fetal colon and rectum measurements from 20 to 36 weeks GA on MRI. Radiologists can reliably measure the normal fetal colon and rectum on fetal MRI. Our nomogram of normal fetal bowel measurements provides radiologists and clinicians with data that can aid in the early recognition of bowel anomalies on fetal MRI.