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Is Your Mouse Pregnant?

The ‘Plug’
Once the male has successfully mated with the female the excess semen forms a white, waxy ‘plug’ in her vagina to prevent other mice mating with her. This is created by particular components of the male ejaculate that coagulate to form a hard plug that blocks the entrance to the vagina. The plug is a coagulum of fluids derived from both the vesicular and coagulating glands, and as such, it can be produced even by a vasectomised male.

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.

Behavioural Changes
Since the hormones in a pregnant female have drastically changed, her behaviour may also change. A usually tame mouse may nip or a skittish animal may seem lethargic. This is fairly normal, but if the change is extreme or if she is putting herself in danger with her behaviour, consult a veterinarian.

Nest Building
To prepare for the arrival of her babies the female may begin building a nest as early as two weeks before delivery. She may use her own nest box, or make her own in a corner or on one of her toys.

Abdominal Swelling
This is the most obvious sign of pregnancy. About a week before delivery her stomach seems to grow and grow and grow! This has been said to look like the mouse has swallowed a ping-pong ball. It is a good idea to remove any ramps, ladders, or toys that she could fall off, as her balance is not as good and she could injure the babies. Also, try to handle her as gently as possible to ensure her safety.

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?
You'll probably want to remove the male once you notice that the doe is pregnant, although you don't *have* to. If this is your first litter you are better off removing him as soon as you know she is pregnant, just to be cautious. If she has female friends in with her leave them in with her, as they will help with the litter. Don't put in new females because you don't want to cause the mother stress.

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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