Though female rats are fertile starting at 6 weeks of age, they are not mature enough mentally and physically to safely carry a litter and take care of the babies to weaning age. Most responsible breeders will agree that 4 months of age is the earliest a doe should be bred from. Not only will her body be physically ready, but you will be able to get a good idea of her personality and her overall health, and know if you even want to breed her. Often behavioural problems such as genetic aggression do not begin to show until 6 months or older so it may be wise to wait and see if any such problems develop.
The oldest a doe should be bred from for the first time is around 8 months. Though it is a myth that a rat's pelvis fuses at this age, rendering them unable to give birth, it is true that the older a doe gets, the more complications can arise from her pregnancy. If you think about it, rats tend to only live on average 2 - 3 years, so at 1 year old, they are middle-aged. Like in humans, older rats have a higher chance of birth defects, small litter size, low birth weight, and labor complications if they give birth for the first time at an older age.
If a doe has already been bred from between 4 and 8 months, she can safely be bred again until she is over a year, health permitting. A doe should never be bred from more than three times in her lifespan, and should always be allowed at least one month's resting period between the weaning of one litter and being bred from again.
Female Reproductive System
The reproductive system of a female is cared for when the rest of her body is cared for, with a proper diet, exercise, and social interaction with other rats and humans. Rats you are planning to breed should especially be taken care of, as both they and their babies to come will require lots of proper nutrition. Make sure your rats are getting a balanced diet with lots of fresh vegetables and fruits added.
Ensuring your female has an intake of folic acid pre-pregnancy can help prevent certain birth defects. Folic acid is found in vegetables such as romaine lettuce, spinach, and broccoli and in fortified grain products like pasta and whole wheat products.
Reproductive Health Concerns
Unspayed female rats are very prone to mammary tumors. If you notice any small lumps on the underside of your female rat, take her to a veterinarian right away. Most mammary tumors are benign and can be easily removed without trouble. However, once a rat has a tumor, chances are it will have more later on in life. Being predisposed to tumors can be a genetic factor, so rats that develop tumors early on in life should not be bred from.
Though rare, female rats can also get bladder and vaginal infections. Talk to your veterinarian if you notice blood in the urine, or blood coming from the rat's vagina when she is not in labor.
Blood or pus from the vagina can also be a sign of pyometra, a condition where the rat's uterus fills up with pus. This is a very serious condition and will be fatal unless the rat is taken to a vet immediately. Pyometra can also present in a massive swelling of the abdomen, with no associated pus leaking out. If you notice your rat getting larger and you know she is not pregnant, this is likely the cause.
Mastitis, an inflammation of the mammary glands, can occur while a rat is pregnant or nursing. If you notice any swelling in one or more teats accompanied by heat, odd-colored discharge, or a foul odor, take her to a vet right away.