Most of this information comes from working with the Sprague-Dawley strain of albino
lab rats.
The female rat's estrus cycle is just four days long, which means that once every four days the rat will experience a period of fertility. Rats will only mate during this fertile period, so one way to track rat pregnancies is to do a simple pap smear and look for sperm on the smear. If sperm are present, pregnancy is extremely likely. The gestational period is 21 days, give or take a few hours.
Infant rats, referred to as pups, are born 8-14 to a litter (typically) and are hairless with closed eyes and ears. They are also incapable of urinating and defecating volitionally, and have very limited thermoregulation. For the first 4-5 days of life, rat pups are entirely dependent upon their dam. All of their nutritional intake is in the form of milk, and the dam cleans them and licks their anogenital region in order to get them to urinate and defecate. Their ability to move around is limited, but they can move around enough to group thermoregulate--when the pups are cold, the litter huddles together, when they get too hot, the pups move farther apart from each other, much like naked mole rats. When separated from their mothers (also referred to as the dam), rat pups will emit ultrasound calls. The dam responds to these calls by finding and retrieving the lost pup.
Pups begin to have the ability to volitionally urinate and defecate around day 5 or 6. Their ears become functional around day 10, which is also when they begin to grow a light fuzz of fur. By day 14 they are able to move around fairly well, and sometimes begin ingesting non-milk foods and fluids on a limited basis. On day 16 their eyes open, and their fur has grown in enough that the skin is no longer visible except on the ears, feet, and tail.
Rats generally stop consuming their mother's milk between day 20 and 25. By the time they are weaned, pups have typically increased in weight by more than an order of magnitude. After this, their food choices are often guided by observing other rats--young rats are more likely to eat something that has been eaten by adult rats. Rats reach sexual maturity by about 60 days of age.
Adult rats spend much of their time grooming and sleeping. Lab rats often have very limited vision, and rely mainly on their sense of smell. Rats are omnivorous, and while they are reluctant to try new foods, they will nibble at unfamiliar foodstuffs, taking only small bites to make sure that they do not get sick. Lab rats typically live a little more than one year.