Female rats come into heat about every 5 days, for about 12 hours. During this short time, if they are bred, they will most likely become pregnant. However, if female rats are exposed to unneutered male rats, they can actually be hormonally triggered to come into heat early, and may become pregnant before they come into heat. Rats are prolific breeders, and can become pregnant very easily. If you don't want your female to become pregnant, be safe and keep her away from males at all times! Mating only takes several seconds to occur.
Female rats begin to cycle around 6 weeks of age, and can become pregnant at that young of an age, though it would be very hard on their bodies. They usually go through menopause between one and a half to two years of age, though some females remain fertile longer, and you should never risk putting a female in with unneutered males, no matter how old she is.
One very important thing to remember is that female rats have a post-partum estrus, meaning that shortly after giving birth (usually within 10 to 24 hours), she will come into heat again, and if there is a male around, she can become pregnant. This pregnancy, however, will usually have a longer gestation period, somewhere around 26-30 days due to hormones related to lactation. The eggs will be fertilized upon mating, but the embryos will not implant for over a week, giving the female time to raise her current litter before turning her attention to her new one. Breeders should NEVER allow the female to become pregnant from her post-partum estrus, as this can be very taxing on her body and can leave the babies unhealthy as well. The post-partum estrus is a good reason to remove the male well before the female is scheduled to give birth.
After the female goes through her postpartum estrus, she will usually not come into heat again for 26-30 days.
How Do You Know When Your Rat is in Heat?
For some female rats, you won't ever know for sure, as they give no sign. But for many, you will notice they are more hyper than usual, and if you gently scratch their back, they will squat down low, lift their tail and flap their ears back and forth. This will only last a few seconds, then they will usually run away. You may also notice that their vulva is more open during their estrus.
Mating
When a male rat is introduced to a female that is in heat, he will commence sniffing her rear end to determine if she is in heat. If the female is not in heat, she will rarely stand for this intrusion, and will run away or sometimes fight him. Most females in heat, however, will allow the male to continue. He will then climb up onto her back and she will lift her tail and allow him to mate with her. Mating takes a few seconds at most, often less than a second. After mating has completed, the male will lick his genitals and the female will retreat. The male will then attempt mating again once he is ready, and this process will repeat many many times. Often the males will tire themselves out and fall asleep with exhaustion!
Though female rats can get pregnant with one mating, it often takes more than one for the male to actually ejaculate, usually the first few matings are courtship displays. Often you will not even see your rats breed, as they are more active during the night, and will usually come into heat at that time.
To ensure pregnancy, it's best to leave the male and female together until you notice that the female is no longer in heat. Do keep an eye on them, though, as some males (even some females!) are rough and can fight and bite if they don't want to mate. If they seem fine with each other, usually leaving them together overnight can ensure pregnancy.
Some breeders like to leave the male and female together for 2 to 2 and a half weeks to make sure they breed. Personally I do not do this, as it can sometimes be difficult to re-introduce a male back to a cage of other males after even such a short time. I've had males I took out to breed for a few days, then tried to re-introduce them and their cagemates attacked them brutally, forcing them to live in a cage by themselves. This all depends on the personality of your males, though, use your best judgement.
Problems With Mating
Some rats are "problem breeders", which can mean any one of a few different things. Sometimes a male or a female (usually a male) can be a rough breeder, meaning he doesn't want to wait for the female to be receptive, he wants to mate with her whether she wants to or not. Of course, when he tries this, the female will fight him and he will fight back, and since males are usually considerably larger than females, he can hurt her badly in just a few seconds. I had this happen to one of my rats once, and the poor female wound up with a nasty laceration on her side that thankfully didn't require stitches. She had let him breed her once, then decided she was done, but he didn't agree. Needless to say, I took him out of her cage, and she didn't end up getting pregnant.
Though this is rare, I have seen it happen that female rats will allow their breeder to scratch them on their back and cause them to perform their "in heat dance", but when a male rat comes along, they are terrified and don't know how to present themselves to him. One female I had would wiggle her ears and stick her rear end in the air whenever I touched her back while she was in heat, but when I stuck her with the male I intended to breed her to, she screamed and ran in circles around the cage, not wanting him to touch her. I put my hand in the cage and scratched her back and immediately she stuck her rump in the air and the male, who was trying to sniff her at the time, saw his chance and quickly mated her! Though she looked very confused at first, she then began to understand what this was all about, and though I did not see him actually mate her again, she was not as afraid, and she ended up pregnant!
