In The Sign of the Beaver, the author Elizabeth George Speare leaves that an unanswered question. When Matt is finally reunited with his family, he notices how frail his mother is. She explains that she has been worried about him. In Chapter 25, Matt goes out to assist his father in unpacking their sled. He inquires about the baby and is told that the baby only lived for five days. There is no indication as to whether the baby was a boy or a girl. "Matt wished he had been able to hide the cradle before his mother saw it."
One indication that Matt never found out if the baby was a boy or a girl is that the baby is referred to with the pronoun it instead of he or she.