Is it true that a Muslim woman who became pregnant before marriage, then married, and her husband died, will not be rewarded for raising and caring for her son, on the grounds that she gave birth to him "illegitimately"?

A woman is sinful for committing adultery, but rewarded for taking care of and raising her child. She must repent to Allah for the adultery, as it is one of the gravest sins. Her responsibility towards the child is the same as that of any other child who has a father.