While You Were Mine by Ann Howard Creel
The cover does not come close to hinting at the story. I probably would have passed it by, except that I loved a previous book by the author.
This story is about a nurse in WWII New York. She takes a roommate to help with expenses. The roommate, newly married to a soldier fighting in Europe, is pregnant. After giving birth, she becomes increasingly troubled, convinced that the lack of word from husband means that he has been killed in combat. She takes less and less interest in baby, becomes more and more distant, until one evening the nurse comes home from work to find her packed and leaving...without the baby or a forwarding address.
Nurse cares for baby, waiting for mother or father to appear at any time to reclaim her, and becoming more attached to her every day.