A comparison of sexing methods in fetal mice.

Accurate mouse sexing is vital when conducting research examining sexual dimorphisms. Late fetal and newborn mouse pups are more immature than many previously described sexing methods allow. This study compares the sexing accuracy of a newly described internal gonad sexing method to a recently described peritoneal pigmentation sexing method in embryonic day 20 C57BL/6J mouse pups, using Sry genotyping to confirm the sex. The internal gonad sexing method was found to be highly accurate, while the peritoneal pigmentation method was slightly less accurate. Therefore, while Sry genotyping remains the gold standard, immediate and less expensive sexing methods can be performed accurately as early as the late fetal period in C57BL/6J mice.