You are more entitled to him as long as you do not get married

In this Hadīth, a woman complained to the Messenger of Allah (may Allah's peace and blessings be upon him) that her had husband divorced her and wanted to take her son from her. The woman justified her right to keep her son with her with the facts that her womb was a vessel for him when he was a fetus, her breasts nourished him when he was a baby, and her lap was a protective place for him.

The Prophet (may Allah's peace and blessings be upon him) approved the woman's description and told her that she was more entitled to the child's custody as long as she did not marry. If she got married, the father would be more entitled to the custody of the child.

The reason for this is that when a woman gets married while keeping her son with her, her son would be under the guardianship of a new husband, who might keep reminding the child of his favors upon him, or the child might feel more attached to him than to his father, etc. It could also lead to other bad consequences.