Mind the Litter: The Critical Role of Intralitter Variables in Reproductive and Developmental Toxicology Research

Animal models remain essential for understanding developmental toxicology, providing insights into how in utero exposures affect fetal and placental outcomes. Litter-bearing species are widely used due to their efficiency and reproducibility, but conventional approaches often summarize outcomes at the litter level, masking meaningful within-litter variability.

This review highlights three primary sources of intralitter variability: litter size, uterine implantation location, and fetal sex, and their influence on fetal and placental growth, mortality, and malformations.

Larger litters restrict fetal growth through intrauterine competition, while toxicant-induced changes in litter size can obscure or exaggerate effects. Implantation location within uterine horns influences perfusion, nutrient delivery, and local exposure to hormones or toxicants, introducing spatially dependent vulnerability. Fetal sex further modifies responses, as male and female fetuses and placentas differ in growth trajectories, gene expression, and adaptive capacity, leading to sex-specific susceptibility to toxicants.

Ignoring these sources of variability risks overlooking subtle effects and vulnerable subpopulations, misinterpreting outcomes, and reducing the translational value of animal studies. We argue that incorporating intralitter variables into experimental design and statistical analyses enhances the accuracy, reproducibility, and interpretability of developmental toxicology research. By refining the use of animal models in line with the 3Rs framework, researchers can maximize the information gained per pregnancy and improve risk assessment for human maternal-fetal health.