A purring cat is not necessarily a happy cat. A cat in great pain, distress or even a female giving birth will purr. A little known fact is that friendly cats that are in pain will purr when approached by people; this suggests that cats purr to show that they are friendly and approachable, ready to be stroked or helped.
At two to three days old a kitten will start to purr, the purr is very quiet and can be difficult for a human to hear. As the kitten grows the purr gets louder. A kitten purring is a signal to the mother that all is well and the kitten is contented and getting enough food when suckling.