Body Length as an Objective Marker of Time to Wean in Laboratory Mice

In laboratory mice, the 21-day weaning standard is the most commonly applied strategy across institutions. However, this strategy has numerous drawbacks, including potential for litter overlap, pup mortality, and weaning extensions. In pursuit of a more objective marker for weaning, we compared the short-term growth of C57BL/6J mice weaned at 3 different body lengths. To recapitulate various weaning scenarios, mice (n = 90) were weaned at a litter average of 5.5, 6, or 6.5 cm. Body lengths and weights of mice were measured twice weekly from weaning to 10 weeks of age. Resulting growth curves revealed no significant differences in body length or body weight found across treatment groups. Although all groups achieved similar adult body length and body weight, unexpected mortality was experienced in the 5.5-cm group (n = 11). Multiple blinded observer comparisons did not result in significant inconsistencies in body length measurements. Our findings indicate that C57BL/6J mice can be safely weaned at an average minimum litter body length of 6 cm. This body length allows for normal physical development into adulthood without the requirement for additional nutritional support. Furthermore, the use of body length is a practical and reliable tool for personnel charged with determining weaning readiness in mice.