Can an unwed mother (single, not divorced) receive Holy Communion? If not, how is her sin different from, say, an adulterer’s?
The matter about which you write is a delicate, pastoral matter, the answer to which would be dictated by a variety of considerations.
If the individual involved is truly repentant and has made a honest attempt to properly raise her child while recognizing that there is a need to prevent such things from happening again, then it is my opinion that she may receive Communion.
If, however, the individual involved sees nothing whatsoever wrong with her situation and, in fact, would go so far as to state that she would do it again, then it is my opinion that she should not receive Communion until she has had a change of heart and mind.
Please understand that, at best, I can only give a general answer based on the very broad question you have asked. It is only my opinion, but any definitive answer on behalf of a specific individual in this situation must be rendered by the individual’s Father Confessor alone.