Discussion
We have established age-specific RIs for small and large intestinal lengths of fetuses and neonates using data in the Eastern Ontario and Western Québec regions. In comparison to our study, prior literature also demonstrates a positive, linear relationship between age and intestinal length RIs.1 2 This holds true for both the length of individual sections of the intestinal tract as well as the total intestinal length. In keeping with other studies, no differences in intestine length were found between sexes, and hence we combined data for males and females to maximize the sample size.4 22 23
Sensitivity analysis showed that the absence of congenital anomalies, gastrointestinal anomalies, maceration, as well as IUGR and LGA fetuses did not influence intestinal length RIs. Unique to our study were findings that fetuses from multiple pregnancies did not impact small intestinal length. The relationship between singleton pregnancies and the quantifiable impact on fetal intestinal length remains largely unanswered. Evidence suggests that fetuses from twin pregnancies are susceptible to a variety of intrauterine complications, such as IUGR, with the incidence of IUGR approximating 25%–35%.26 27 Whether IUGR impacts fetal intestinal length remains equivocal,3 4 and in our analysis IUGR did not have any impact on intestinal length. Similar to prior literature,4 maceration did not appear to have an impact on the growth of small intestinal lengths, and is in keeping with prior literature demonstrating that maceration does not alter all body measurements.15 The presence of congenital anomalies did not alter the small intestinal length. Though prior literature has demonstrated a reduction in length in Trisomy 21 fetuses,4 we included a range of congenital anomalies, such as Turner syndrome and vermian hypoplasia, in addition to Trisomy 21. Combining the congenital anomalies may have attenuated the veritable impact of Trisomy 21 on fetal intestinal growth. Unfortunately, due to sample size limitations, a subgroup analysis was not possible and represents an area of further research.
Capturing fetal intestinal length as early as 13−41 weeks is useful for both surgeons and pathologists to visualize intestinal length across a wide range of gestational ages. From a clinical standpoint, access to intestinal length RIs across a range of ages may assist in refining prognosis and treatment plans for preterm or term patients where intestinal resection is necessary, such as for Hirschsprung’s disease. Pathologists analyzing biopsies for transition zone between aganglionic and ganglionic cells can communicate residual bowel length to surgeons,28 which could assist surgeons in further planning postoperatively. Residual small bowel length may allow surgeons, for example, to predict the ability of an infant with short bowel syndrome (SBS) to wean from total parenteral nutrition following bowel resection.29 Furthermore, defining accurate small bowel length may provide benefit to pathologists conducting postmortem examinations. As mentioned earlier, there is currently a wide range of definitions being applied to SBS in fetuses and neonates, and establishing RIs may help apply this definition more precisely among this population based on age. Additionally, intervals may assist pathologists in identifying the pathological bowel length and in deciding when to screen for genetic anomalies, such as the CLMP gene in congenital SBS.11 13
Owing to limitations of sample size, determining reliable estimates for intestinal length is difficult. Wide variability in intestinal length observed within partitions in our data may indicate the need for a per-week analysis to capture minute differences over time because intestinal growth can vary widely in fetuses. Marnerides et al established fetal intestinal length intervals in the second trimester and found considerable differences across weekly partitions.4 To overcome these challenges, reference curves may be fitted to data and normal ranges can be obtained by using the functional form of the estimated curves at a given gestational age. We plan to pursue this approach in a subsequent study.
There are several limitations associated with this study. First, bowel measurements were made postmortem after removal of the bowel from the abdomen. Postmortem measurements can overestimate bowel length due to initial muscle contraction shortly after death, when relaxation and loss of tone occur as autolysis begins6; however, we did examine intestinal length in the sensitivity analysis without maceration to account for this. We did not find any impact of maceration on intestinal growth. In contrast, use of silk sutures to measure intestinal length in live infants provides a promising method to accurately measure intestinal length in vivo, although this methodology requires further external validation.19 We also followed a routine protocol as described in similar studies1 by laying out specimens on a moist plastic surface to minimize ‘stretch artifact’, and the same experienced perinatal pathologist measured each specimen twice to reduce error in measurement. Hence, the variations in gut length are not likely to be attributed to measurement inaccuracy. An additional limitation involves our population of study because the results are based on postmortem fetuses. Our results are thus not truly representative of the normal fetus population. However, the RIs could still be applicable to postmortem populations with similar characteristics.4 Further, to develop statistical models that account for the effects of age, sex, congenital anomalies, and other covariates, the Hoq et al’s method could have been used to estimate RIs as a function of age.30 The methods we used by Daly et al21 were valid but did not exploit the linear relationship between age and intestinal length. We also partitioned the age categories in a narrow manner because measurements increased linearly with respect to gestational age. However, this approach may be seen as subjective and contingent on the sample size available in each age category. We will use the discrete age groups commonly reported in the Hoq et al’s30 systematic review to address this limitation in a future study.
To conclude, this study has provided age-specific RIs for normal small and large intestinal lengths of fetuses and neonates. Pathologists can compare our data to other studies that have established reference standards for normal intestinal length. From a surgery perspective, understanding the expected or normal intestinal length might aid surgeons to communicate how much intestine is remaining in infants with parents and to plan postoperative management and prognosis accordingly. Following the standard protocols for conducting linear measurements of gastrointestinal contents,2 31 future studies are encouraged to replicate the study methodology at other health centers for cases in their catchment areas to enable comparisons across provincial institutions. More importantly, further research can be conducted to examine additional factors, such as gestational weight on intestinal length, using the Hoq et al’s30 method and to compare such results to established RIs. This will aid in better understanding of the differences in intestinal length between normal and pathological fetuses.
Lastly, coordination of multicenter, nationwide efforts to collect small and large intestinal length data (eg, similar to the Canadian Laboratory Initiative on Pediatric Reference Intervals)32 and to establish RIs in accordance with CLSI guidelines is needed.