Using postnatal age to determine test dates leads to misinterpretations when treatments alter gestation length:...

A collaborative study was conducted by researchers from 18 laboratories that participated in the Behavioral Teratology Meeting in Japan. Pregnant Sprague-Dawley rats from four breeders received subcutaneous injections of nicotine (6 mg/kg body weight) from day 7 to day 20 of gestation. Results of preweaning tests were closely related to length of gestation, and prolonged gestation was seen in the nicotine group. The effects of nicotine were compared with and without the adjustment of the mean difference in gestational lengths. Without the adjustment (i.e., by employing assessment in terms of postnatal day) several perplexing results were obtained, indicating that the nicotine group developed more quickly than the control group on several preweaning tests. By employing the adjustment, these perplexing results disappeared, indicating that the nicotine group developed more slowly than the control group. The merit of employing gestational day (or postcoital age) as an alternative index is emphasized.