Is Your Mouse Pregnant?
Usually the plug is visible through a simple visual examination of the vulva. After a few hours it is expelled and you may find it in the cage. This almost always means that your pet is pregnant. Most plugs will persist for 16-24 hours after copulation. Plugs formed by outbred mice can persist for several days.
This blue doe is heavily pregnant
The normal gestation period (the time that the babies are inside the mother's womb) is about 21 days for a mouse. If you know when she got pregnant, it will be easy to calculate her due date. Once her abdomen begins to swell immensely she probably has a week to a week and a half left in her pregnancy. It is then that you must begin to prepare for labour.
It is possible to identify a state of pregnancy in young females by a simple visual inspection that does not even require one to handle the animal. The gestational day at which this becomes possible is greatly dependent on a number of factors including the age of the female, the number of foetuses inside and whether she has given birth previously. For first-time pregnant females carrying large litters, tell-tale bulges from the centre of her body can be detected by day 15. At the opposite extreme, older multiparous females with small litters never 'show' in this way. Fortunately, these older animals are easier to palpate when a prenatal determination is required.
Self black doe at 2.5 weeks pregnant with nine pups
Dove banded doe three weeks pregnant with 14 pups
'Feeling' the Babies
Later in the pregnancy, from 10-12 days post conception and beyond, it becomes possible to feel the maturing foetuses within the uterus by simple palpation. Pregnancy palpation is most readily carried out on older, multiparous females who have looser skin and are more accustomed to being handled. When the female is securely held by one hand and relatively calm, one should use the other hand to close down firmly on the abdomen close to the spine on one side at a time with the forefinger and thumb and then gently move the fingers out. Initially, a pregnant female will seem to have a string of beads on each side of her body. As development proceeds, these "beads" will mature into larger, more defined shapes. With experience, this method can be used to determine the gestational stage of a pregnancy to within a single day.
My Mouse is Pregnant. Now What?
Gestation is 21 days, but the most noticeable growth in the pregnant female is generally in the last week. She will grow a large lump in her middle and look distinctly pear shaped. Give her a nesting box big enough for her and all her babies and the nesting material. Half a tissue box will work well in the absence of anything else suitable. Put in some nice bedding (e.g. strong tissue paper) - I usually put in soft tissue and a bit of special nesting material I get at the pet shop. Make sure she has lots of food and water. You may want to mix her water with vitamins to make sure she is getting everything she and the babies need and you should give her fatty food or extra treats. Some treats mice like are cereal (but not those containing wheat), dry bread, cooked whole grain rice, cooked beans, millet, corn flakes, cooked turkey or chicken, scrambled egg and other lactose-free snack items.
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