A female has a litter (this term can be used for all mammals whether or not the births are multiple but it is usually non-PC to use it for humans), and the offspring of rats are also called kittens (in multi species environments such as the records of an animal shelter the term “rat kitten” may be used to avoid confusion with felines)…

the oestrous (sp?) cycle of female rats (and their behaviour) is better described here

National Fancy Rat Society