Having babies is very hard on a mouse. It may seem that mice, or any other animal, are built for having babies but that's not the case here. Some females can even die during delivery. Breeding from well tracked lines drastically increases your chances of having a successful litter. When deciding if you want to breed your mice make sure you consider that.
Mice *can* get pregnant as early as 4 weeks. However, it is unbelievably hard on them if they do. Never breed your mouse that young, no matter what. In addition, make sure you separate your babies (males from females) at 4 weeks of age so this doesn't happen. It is generally recommended to wait until the female is 12 weeks old or older before she is bred for her first litter. Breeding sooner is bad for the mother and the babies. It can cause weak mice as well as shorter lived mice.
Depending on the line, a female shouldn't be bred much past 8 months either. As stated before, having babies isn't easy for a mouse. By the time that they are 8 months old they are actually getting elderly! Breeding age will vary from line to line. Some can be bred older and others can't be bred that old. If you don't fully know your line, don't breed past 8 months. Breeding a doe too late in life will almost always result in death.
A doe should have time to recover in between litters. It is not good for her to be bred back to back. A mouse can conceive another litter after giving birth (postpartum estrus)! Be sure the male isn't with the female when she has her babies. A mother mouse should have a 3-4 week recovery period, beginning after she has weaned her litter, minimum. The longer you wait the better. Keep in mind that the closer you breed her to her last litter the chances of complications increase. Complications can range anywhere from a difficult birth, small litters, unhealthy litters and/or mother, shorter life spans of the mother and/or babies, death of the mother and/or babies, among many other problems.
A doe should NEVER have more than 3 litters in her life time. Some does can't even handle that many. Mice are not breeding factories. They are little lives and their best interest should be in mind when breeding. Breeding a mouse too many times will always result in shorter life spans of the mother and babies. It will also cause them to be less more vulnerable to health issues. It can also easily result in death.
Mating & Conception
It is best that you introduce a breeding pair the same as you would introduce any mouse to another mouse (refer to the section on Introducing mice). Make sure their home is large enough for both buck and doe. Also make sure that they have enough toys, a big enough nest, food, water, etc. Their house should be fully cleaned and a neutral territory. While uncommon, sometimes bucks and does will fight. Placing a doe in a bucks home (or vice versa) can easily cause a fight. If you just place a doe with a buck, all the buck sees is another mouse invading his territory. This is the equivalent of a stranger barging in your house. Mice, like people, are more likely to react and ask questions later. It is only instinct to protect ones home. Also know that mice have extremely poor eyesight so they have no idea what is actually invading their space. It could be another buck come to fight with them for all they know. It isn't their fault and it does NOT necessarily mean that they are aggressive. They just need to be introduced properly so neither mouse feels threatened. As with any introduction, the buck and doe should be watched for quite a while after placing them together to insure that a fight doesn't happen. There are some aggressive bucks that will fight with a doe no matter what. This is very rare but does happen. There are also some does that will fight with bucks. This is generally caused by an overly aggressive buck which forces the doe to take more aggressive measures to prevent breeding when she is not in heat. It is normal for a doe to push a male away when she is not in heat. This can often involve slapping and a lot of squeaking. However, if fighting occurs you should separate the breeding pair. If they are not separated severe damage can occur, as well as death. While extremely rare, does will attempt to neuter a buck if he is overly aggressive, or if she is overly aggressive. It is not completely unheard of to have a doe rip open and possibly remove a males' testicle. If this occurs, emergency vet attention is needed. If either mouse is overly aggressive you should NOT use him or her (or both) for breeding, ever. Again, this is extremely rare but you should be aware of even the most remote possibilities.
In addition to leaving the buck and doe together for 16 days to help insure pregnancy, there are other ways you can monitor whether a doe is pregnant or not. You can monitor your does heat cycles before attempting to breed her. A doe will come into heat every 3-5 days, normally. However, some does might miss cycles, some may not cycle regularly, etc. Monitoring heat cycles isn't easy though. A mouse is only in heat for 12 hours. You can easily miss her heat cycle if you don't check at the right time. If you check your doe often and catch her in heat, mark it on your calendar so you know her individual cycle. Do this for at least a month to get an idea of how she cycles. When you are breeding her, knowing her cycle will help you predict when she is due. It will also help you to know if she is pregnant. A pregnant doe will not come into heat again until after birth. If your doe is missing her cycle after she's with the buck, she's likely pregnant. To know if your doe is in heat her vagina will be opened. It may also be swollen and red.
When a buck mates with a doe he will leave behind a vaginal plug as shown in the picture on the right. This plug is designed to prevent other bucks from mating with the doe. The vaginal plug looks like white mucus. It is a liquid-paste form at first, then hardens into place. Sometimes this plug can be seen and sometimes it can not. The plug may be too deep to see. If you are able to see the plug, it can help you know when the buck mated with the doe. However, a plug does not always mean that your doe is pregnant. A plug doesn't confirm pregnancy, it only confirms mating has taken place. Having a scale to monitor weight can give you a better idea if she is pregnant or not, but it's not 100% effective. Females will often lose a couple/few grams within a day or 2 after mating has taken place, then start rapidly gaining weight the further they get into their pregnancy. However, sometimes mice just gain weight, possibly making you think they are pregnant when they are not. Some mice don't gain much weight at all during pregnancy, making you think they are not pregnant when they actually are. Scales can be a useful tool, but like anything else they don't insure pregnancy.
All of these tools are useful when breeding. However, none of them are 100% effective. There is no way to know that your mouse is pregnant until she has babies. Some ailments can resemble pregnancy, so even if a mouse looks pregnant it doesn't necessarily mean she is.
It has been reported that a female mouse can abort her babies if she smells another male mouse other than her mate. This is extremely rare with domestic mice. However, studies show that a female mouse will abort and absorb her fetuses anytime after being 2 days pregnant if she smells another male mouse other than her mate. The cause of this is pheromones. These pheromones send off signals to other mice. In this case it's a male mouse who's telling a female that there might be a better suitor for her. Since she wants her babies to have the best chance at survival, she must mate with the strongest male or the "alpha male." To do this she must abort the litter she is carrying first. It is doubtful that this will happen if there are males in other tanks around the pregnant doe. However, it is very important to never allow another buck near your pregnant doe for this reason as well as knowing the who father is.
Pregnancy
Some pregnant mice look like they swallowed a golf ball while other mice don't look pregnant at all. Some mice look exactly the same from the time they become pregnant to the time they deliver. This is why it is so important to separate the mother from the father before she could possibly have babies. Don't let her size, or lack there of, fool you. The 2 pictures shown above are of a mouse that has the "I ate a golf ball" look. She is in her last few days of pregnancy. Not all mice look like this. Some are bigger, some are smaller. Some mice look huge when pregnant with very few babies while others with much larger litters might not look pregnant at all. Some mice have small litters with large size babies while other mice have large litters with small size babies. Some mice have average litters with average size babies too. The size of your mouse is not an indication of how many babies she has.
While it is possible to feel the babies in your mouse, do NOT attempt this. You can easily injure the babies and the mother. Killing a baby inside the mother is extremely easy to do. If this happens, it also kill the mother if she isn't able to absorb the fetus. This does not mean that you can't hold her and give her attention though. If the mother isn't looking too uncomfortable and she wants attention, you may absolutely hold her. Let her climb on your hand. Make sure all 4 of her feet are supported by your hand and then lift her up. She will regulate the pressure on her abdomen. Never pick a pregnant mother up by the belly or the tail. Picking up a mother by the tail during her pregnancy can cause the tail to break, fall off entirely, or pull the skin right off. Their tail is not a handle and should not be used as such.
Do note, however, that a mouse with worms can look exactly like a pregnant doe. If this is the case, the worms are severe and immediate vet attention is needed! Additionally, a mouse with a blockage in the digestive system can also look like this as well. Again, if that is the case immediate vet attention is needed. Lastly, some chubby mice can look pregnant. If this is the case a better diet and exercise is needed.
Diet for pregnant and nursing moms.
Mice in general shouldn't be given high protein and high fat diets. However, pregnant and nursing moms need additional protein and fat at this critical time. Boosting their diet should start half way through pregnancy and last until after they wean their young. You may boost them before breeding, however, you should never boost them nearly as much as you would a pregnant and nursing doe. Giving a pregnant doe one plain small puppy biscuit (such as Milk Bone Puppy biscuits) once a day works well. Other good things to give them are cooked eggs (scrambled), small pieces of cooked meat, puppy or dog kibbles, and oatmeal. Remember to remove any food that can spoil (such as meat and eggs) within 2 hours after you first offer it. Leaving it out longer can cause food poisoning and death in mice. Always give these things in moderation as well. Giving them too much can do more harm than good! Additionally, if a mouses' diet is suddenly changed it can cause major digestive upset. This will make her sick and likely cause runny stools. A mouse with runny stools can dehydrate extremely quickly. A dehydrated mouse can die within 24 hours. A few loose stools isn't anything to be too concerned about. This can be caused by a number of things, many of which are not a cause of concern. However, constant runny stools are a large cause of concern. It is best to slowly get your mouse used to these foods. It is also wise to know what types of food your mouse can't handle before breeding and diet boosting takes place. Giving a mouse something new can cause an allergy which is extremely worrisome during pregnancy. Other than this your mouse should be fine on her normal diet prior to getting pregnant, as long as your mouse is on a good staple diet to begin with.
Housing pregnant females together.
Sometimes keeping to pregnant females together works out great when other times there are devastating results. The up side to having 2 moms together is that they can share the responsibility. Both moms may take an active role in raising the young. IF all goes well, having 2 moms together is a lot easier on them because of how much they can help each other. This is especially true with first time moms. Having 2 of them together is like having their own support system.
However, sometimes theft does happen. Some moms love being a parent so much she will take both litters for herself. At least both moms have milk and can take care of them, unlike theft among nannies. However, depending on litter size, it is doubtful that the mother will produce enough milk to adequately feed both litters by herself. This can cause a weak mother as well as weak babies.
Sometimes abandonment of one litter can happen. Sometimes both moms will not have babies on the same day. If there is more than a couple days in between the births, the younger litter might be abandoned. The mom that had her babies second will see that her babies are a lot smaller than the first litter. She may think that her babies are unhealthy because they are so small. At that point she may either abandon them or kill them. Nature has a way of weeding out the weak and in this case the mom is confused. Even a couple days makes a very noticeable difference among baby size. See the day-by-day baby pictures on this site for reference. If the babies do get abandoned you can try to save them by placing the abandon litter with the other litter or hand raising the babies. Before merging the 2 litters together you must know that there is a big risk in doing so. It is possible that both moms may abandon both litters or that one or both moms may kill both litters. If you decide to introduce the younger litter to the older litter, take both mothers out of their tank and then place the younger litter at the bottom of the pile of babies. Then put both moms back and leave them alone. If you choose to hand raise the litter, see the Orphan babies section of this site.
When allowing 2 does to have their litters together, it is often unlikely that you will know which babies belong to which mother unless you have a good understanding of mouse genetics and plan the litters extremely well. Surprises often pop up in litters, making it very hard to know which mother the surprise babies belong to if they are birthed together. Extremely careful planning needs to be made to insure this doesn't happen. You need to know the exact genotype of all parents involved. It is best to breed parents where you are sure what the outcome will be, such as breeding a homozygous agouti parent in one litter (resulting in 100% agouti babies) and breeding 2 non-agouti's for the other litter (resulting in no agouti babies). However, if you end up with something such as PEW, you will likely not know -for sure- which mother those babies come from. When breeding, it is extremely important to know what babies belong to which parents. This insures well tracked lines.
If you choose to take the risk of letting 2 does have litters together, there are some precautions you can take to help the minimize risks. It greatly helps if both does are the same age and in the younger part of their optimal breeding age. This helps insure that their heat cycles are more regular. House the does together for months before breeding them. This will help get their heat cycles on the same track. Females of any species "pull" each others heat cycles and they tend to go into heat about the same time. If they go into heat at the same time, they have a better chance of conceiving at the same time, in turn increasing the chances of them having babies at the same time. It also helps to breed both does to the same buck so he can impregnate them around the same time. Different bucks might breed differently. Some (most) bucks are happy to breed as soon as the doe comes into heat. However, some bucks are lazy and it takes them a while to actually breed the doe. Some bucks also have lower sperm counts than others. This can make it harder for one to impregnate a female.
source: http://www.thefunmouse.com/info/breeding.cfm
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