Breeding Considerations for Mice

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Female mice should not be bred before 50 days of age.  They are

continuously polyestrous, which means that they come into heat

at fairly regular intervals (every 4-5 days) throughout the

entire year unless they are bred.  The period during which the

female is receptive to the male and allows breeding is about 12

hours and usually occurs at night.  Female mice can come back

into heat within 14-28 hours after giving birth to a litter.

This is called a postpartum estrus, which means that they can be

nursing a litter and pregnant at the same time.



Pregnancy lasts an average of 3 weeks but can be extended as much

as 10 days longer if the pregnant female is suckling a previous

litter.  Litter sizes average 10-12 pups, though it is not

unusual for a female's first litter to be smaller in number.

Litter sizes decrease as breeding females age.  Though mutilation

and cannibalism of the young are rare, it is wise not to disturb

mice for the first 2-3 days after giving birth.  Pups are usually

weaned at about 3 weeks of age.  The female resumes her breeding

cycle 2-5 days after her pups have been weaned (unless she was

bred during her postpartum estrus).