The age of a mouse embryo is typically presented in gestational days or as the number of days post-fertilization (days post coitum, or dpc). The half-day demarcation is due to the typical breeding procedure with mating set up in the late afternoon, with noontime after a vaginal plug is detected in the morning, being defined as 0.5 dpc. The mouse is typically born about day 19 post-fertilization.