The importance of specification of minimum age requirements of female age at mating to achieve regulatory toxicology study objectives: An evaluation of age at mating on varying study types and correlation to litter size in three strains of laboratory rat

ETS 2025 -- Litter size is a critical endpoint on developmental and reproductive regulatory studies in rodents, as rodents are the species of choice due to larger litter sizes compared with other species. There is some drive from animal suppliers to mate animals at a younger age than recommended by certain test guidelines, which may potentially be suboptimal for achieving study objectives. Control data from rat studies performed at two Labcorp sites were assessed to see if there is a correlation in the approximate age of females with mean litter size. The OECD 414 test guideline (2018) does not specify an approximate age at mating. OECD 421 and 422 test guidelines (2025 [para. 30]) states study starts (estrus cycle evaluations to be start around 10 weeks old for Sprague Dawley® (therefore 14 weeks at mating) or 12 weeks Wistar® Han rats (therefore 16 weeks at mating) taking into consideration a 2 weeks predose estrus cycle assessments and 2 weeks pre pairing. OECD 443 (2025 [para.11]) states that the parental generation should be approximately 90 days at mating, and F1 generation mating to begin on or after postnatal day (PND) 90 but not exceeding PND 120. The ICH S5 (R3) pre- and post-natal study: F1 generation is paired at least 10 weeks of age for mating.