The charming, curious and social domestic mouse has been kept as a pet for centuries, according to the ASPCA. Thanks to her prolific breeding ability, the mouse population is distributed throughout the world. A female mouse will typically give birth to 42 to 60 offspring per year with five to seven mice in each litter. Gestation generally lasts 19 to 21 days. Knowing how to tell when your mouse is pregnant can give you time to prepare.
All Plugged Up
After mating, the male mouse leaves excess semen at the opening of the female's vagina, which creates a hard white plug. This "plug" is nature's way of preventing other males from mating with her. If you examine your mouse's vulva area, the plug is usually visible. The plug will typically remain in place for 16 to 24 hours after mating, but can remain in place for up to 48 hours. When the plug is expelled, you might find it in your mouse's cage, which is a sign that she could be pregnant.
Preparing for the Little Ones
To prepare for the arrival of her offspring, the pregnant mouse will begin building a nest around five to 14 days before giving birth. Provide her with a nesting box that's large enough for her and the babies, such as an empty tissue box. You can also leave material in her cage so she can construct her own nest, such as shredded paper, cloth, string, plants, burlap or other soft material. She'll use the material to create a loose ball around 4 inches wide, typically on top of one of her toys or in a corner of the cage.
One of the most obvious indicators of pregnancy is a bulge in the center of the female mouse's body and enlarged nipples, which typically are visible by day 14. Your mouse will develop a more pear-shaped appearance. The exact day that a larger abdomen becomes visually apparent is dependent on the number of fetuses the female is carrying, her age and if she's previously given birth. Older females with past pregnancies might not develop noticeably larger abdomens, particularly if they're carrying small litters. Remove any toys from the cage that might cause the pregnant mouse to fall and injure herself or the developing babies, such as ladders and ramps.
The Happy Day
Mice are social creatures and take better care of their offspring when they're surrounded with harmonious cage mates. Other females will typically help the new mother raise and care for the litter. Avoid adding new mice to the cage immediately before or after birth, as the new arrivals can cause stress to the mother. During the first day, avoid disturbing the mother and her litter. By the second day, her maternal instincts will have kicked in and she won't become so rattled by disruptions. Keep the cage in a peaceful and well-ventilated area of your home without loud, disturbing noises.