It is very important to pick a healthy female - after all, she will be doing all the hard work of getting through pregnancy and raising the litter. Also, you need to make sure that she will pass on healthy genes and a good temperament. Another important thing to consider is, does she really want to be a mom? There are some rats who just know that they should not be bred, and then there are others who hang teasingly all over the guys' cage the minute they hit puberty! Listen to your common sense and your rats, and you should do just fine.
If the female is being bred for the first time, there's a pretty short window of time when she can be bred safely. If she's too young, she'll be a very nervous, flighty mother who may lose a baby or two. If she's too old, she might risk losing her life and the life of all her babies. Before the age of 5 months or so, females are still growing up, and won't do well with the responsibility of childbirth. Some females make good moms at an early age, but it's best to let them grow and develop fully before putting the stress of motherhood on them. But don't wait too long; between 8-10 months, female rats' hips finish fusing together. If they haven't been bred before this age, breeding after it can be deadly. When the hips fuse the birth channel can become too narrow for the babies to get out, and both mom and kids can die. If you suspect that an older female is pregnant, it's a good idea to be in contact with a vet you trust, who will be available to perform a cesarean in case of emergency. Females who have already had one litter have wider hips and can be bred safely later on, but never, never, never breed a rat older than 8 months for the first time.
