The average gestation period is 20 days. A fertile postpartum estrus occurs 14–24 hours following parturition, and simultaneous lactation and gestation prolongs gestation by 3–10 days owing to delayed implantation. The average litter size is 10–12 during optimum production, but is highly strain-dependent. As a general rule, inbred mice tend to have longer gestation periods and smaller litters than outbred and hybrid mice. The young are called pups and weigh at birth, are hairless, and have closed eyelids and ears. Pups are weaned at 3 weeks of age when they weigh about . If the female does not mate during the postpartum estrus, she resumes cycling 2–5 days post-weaning.