Editing Laboratory mouse (section)

== Reproduction ==
[[File:Day-old mice.JPG|right|thumb|1-day-old pups]]
Breeding onset occurs at about 50 days of age in both females and males, although females may have their first [[estrus]] at 25–40 days. Mice are polyestrous and breed year round; ovulation is spontaneous. The duration of the [[estrous cycle]] is 4–5 days and lasts about 12 hours, occurring in the evening. Vaginal smears are useful in timed matings to determine the stage of the estrous cycle. Mating can be confirmed by the presence of a [[copulatory plug]] in the vagina up to 24 hours post-copulation. The presence of sperm on a vaginal smear is also a reliable indicator of mating.<ref name=lvma />

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 (animal)|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 (biology)|hybrid]] mice. The young are called pups and weigh {{convert|0.5|-|1.5|g|abbr=on}} at birth, are hairless, and have closed eyelids and ears. Pups are weaned at 3 weeks of age when they weigh about {{convert|10|-|12|g|abbr=on}}. If the female does not mate during the postpartum estrus, she resumes cycling 2–5 days post-weaning.<ref name=lvma/>

Newborn males are distinguished from newborn females by noting the greater [[anogenital distance]] and larger [[genital papilla]] in the male. This is best accomplished by lifting the tails of [[littermate]]s and comparing [[perineum|perinea]].<ref name=lvma/>