Though there are many ways to determine pregnancy, very often rats do not show any signs of pregnancy until shortly before birth. If you were not intending to breed your rat, it's very possible to not notice a pregnancy at all. Here are some signs to look out for, in the general order in which they can be seen.
No More Heat Cycle
When a female rat is pregnant, her heat cycle is put on hold until after the babies are born. If your doe was obvious about her heat (see estrus and mating section), this may be a good indicator of pregnancy. If you know when your female last came into heat, or you witnessed the mating, you can check to see if she is in heat by scratching her back gently to see if she presents herself for mating, and you can examine her vulva to see if it is more open than usual. If you do not notice her come into heat for over a week after she was bred, and she normally showed signs of being in heat before, chances are she is pregnant.
Weight Gain
Until your rat is close to giving birth, you probably won't notice any weight gain just by looking at her. This is where a correct pre-pregnancy weight comes in handy. Using a small gram scale, weigh your rat before pregnancy, and then once you suspect she is pregnant, continue to track and record her weight. If she is pregnant, you should see a slow increase in weight, with a much larger increase towards the end of her pregnancy. Keeping records of your doe's weight is also a good idea because you will be able to tell if she lost her litter.
Hair Loss Around Nipples
During the last week of pregnancy, the female rat will lose some of the hair around her nipples, making it easier for the kittens to find them. This will make her nipples more noticeable than before and is a definite sign of a litter to come.
Building a Nest
Most rat mothers begin to build their nest a few days before the babies are born, though some prefer to do it after. Though even non-pregnant females exhibit nesting behaviour, you will notice that a pregnant female's nest is usually more elaborate. Often they will pile all the bedding into one corner, or rip down hammocks to use as nesting material. Make sure you provide safe and adequate nesting material at this stage, or the babies may get cold or hurt. I like to use small squares of polar fleece, as they are soft, don't fray and leave dangerous threads, and is fairly inexpensive.
Swelling of the Abdomen
During the last few days of pregnancy, if the female is carefully observed, babies can be seen kicking at her sides and moving about inside her. You can also feel lumps along her side and belly, and sometimes the lumps can stick out pretty far if a baby has turned to one side. Though you can palpate the babies at this point, it is not a very good idea, as they are very tiny and fragile, and accidentally squeezing too hard can cause serious damage. If you are interested in how many babies she will be having, see the next topic.
Unlike dogs and cats, rats should not have their abdomens palpated, and is even dangerous for an experienced veterinarian to do so since the babies are very fragile and not fully developed. Here are a few ways to estimate the number of babies your doe is carrying.
Tracking Weight
If you have been weighing her every day or two throughout her pregnancy, you will have a good idea as to how many babies there are. Each baby rat weighs about 5 grams (0.18 oz) just before they are born, so based on how much weight your rat has gained (keeping in mind the weight of the placentas and amniotic fluid), you can guess how many babies she is going to have. This is the most accurate way to guess, though it can still be quite inaccurate.
The Doe's Age
A doe of good breeding age (4 months to 8 months) will have an average sized litter of 8-14 babies. Of course, for a number of reasons, a healthy doe of breeding age may have much more or much less than that. But if your female is younger than 4 months, or older than 8, or especially older than 1 year, chances are her litter size will decline significantly.
Genetics Can Play a Role
Some types of genes can cause babies to be reabsorbed while still in utero, such as the pearl gene (Pe). It is a dominant gene, and if two pearl rats are bred together, the babies that receive two copies of the dominant gene (PePe) will be reabsorbed before they are born, resulting in a smaller litter size.
Pregnancy Behavioural Changes
A pregnant rat's behaviour can change in many ways, from not at all to very drastically. A female who was normally calm may become hyper and obsessed with building a nest. A female who was normally tame may nip at you every time you come near her. This is usually normal, and caused by pregnancy hormones. This behaviour usually disappears once the babies are born, though some does are very protective of their babies, and the biting behaviour doesn't go away until after the babies are weaned. What behavioural changes your female goes through depends on the situation, just keep a close eye on her and make sure she is not doing anything to harm herself or the babies once they are born. If this behaviour does not change shortly after the babies are weaned, it would be advisable to not breed from this doe again.