Baby rats become fertile at about 5 weeks of age so they need to be kept separate from even before this time. For a planned mating, pair the female that’s in heat with a fertile male in the same cage overnight. It is not advised to leave them together after mating has taken place as the buck will quickly become ‘whipped’ and unable to socialize with his male buddies again very easily, just like humans.
The other reason is that does go into heat immediately after giving birth so it’s likely that she would get pregnant again. This would put a lot of pressure on her to suckle the newborns while pregnant for the second time.
Does need to be separated from their cage mates when fully pregnant. This would be ideal at about 20 days of gestation. The mom-to-be needs a comfy, dark box with lots of nesting material, not too much light and plenty of privacy. This makes the doe feel really relaxed and more likely to have a successful delivery. Does are great at giving birth alone and will clean up and take care of the babies perfectly fine without complications. If there is a deformed baby or one is born dead, the mom will most likely eat it.
Take note that if the labor takes longer than 3 or 4 hours, there might be a problem and a vet needs to be called. They can perform a caesarian but don’t expect the babies to survive.
If you are considering letting your rats breed you need to be able to either take care of them all yourself, or find them good homes. Selling them to a pet store is a gamble since they might end up as rat food or they could just not be interested in taking any more. To minimize the chances, only sell to stores with impeccable reputations as they will most likely sell them as pets.
It’s usually best to only breed the most healthy, cute and extroverted rats as these traits will then most likely be passed on to their offspring. This increases their chances of selling fast as well.
A great resource on how to breed rats well is a book called ‘The Proper Care of Fancy Rats’ by Nick Mays.
You could also contact your rats breeder and get their opinion on how to go about it. Make sure that not only are both the buck and doe particularly good genetic specimens, but also that the doe is at least 4 months old so that she has matured somewhat. A doe needs to give birth for the first time before the age of about 8 months to prevent complications. If she has already given birth before, this time can be extended to about a year.
The mother and her newborns must be left together until they are about 4 or 5 weeks old. This ensures they are fully weaned. After about one more week they are ready to go to a new home as long as they are healthy, happy and have no complications. The mother should be left to recover for about a month before she is allowed to breed again.