A critical look at adjusted fetal weights in rats

• Published in: Birth defects research Vol. 116; no. 1; pp. e2278 - n/a
• Main Authors: Wise, L. David, DeSesso, John M.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01-01-2024; Wiley Subscription Services, Inc
• Subjects: adjusted; EFDT study; Familiarity; fetal weight; Fetuses; Literature reviews; Litter size; normalized; rat; Search engines; Statistical methods; Statistics; Toxicity; Toxicology; Weight; EFDT study; rat; normalized; adjusted; fetal weight
• ISSN: 2472-1727; 2472-1727
• DOI: 10.1002/bdr2.2278

Background
A new derived (i.e., calculated) endpoint of developmental toxicology has appeared in a very few studies since 1990. This endpoint is adjusted mean live fetal weight per litter or adjusted fetal weight. Given our lack of familiarity with the endpoint, we evaluated the basis, prevalence, methods, and usefulness in embryo‐fetal developmental toxicity (EFDT) studies in rats.
Methods
Literature searches were performed with key terms using PubMed and Google Scholar. Major textbooks were consulted but lack of any mention of the endpoint. Unpublished EFDT data, which are readily available online, were utilized to test adjustment methods.
Results
Pertinent information on factors that influence fetal weight goes back a century. Four papers utilizing rats were found in which fetal weights were adjusted using either statistical or formula‐based methods to adjust fetal weights. Only one study showed a clear benefit to the endpoint when there was a marked decrease in live litter size; this pointed to situations in which the new endpoint might be useful. The lone formula‐based adjustment method was found to be lacking adequate testing and justifications. A new experimental alternative formula‐based adjustment is shown to produce results very similar to statistical methods.
Conclusions
From this assessment, we recommend that adjusted fetal weight should not be a routine endpoint at this time. However, there are likely cases where this derived endpoint could aid interpretation. We encourage other investigators to examine previous EFDT study data to establish guidance on the use of adjusted mean live fetal weights.