A more thorough explanation:

Definition: A mother is a woman who has given birth to a child, provided the egg for a child, or legally adopted a child. This term can also include a pregnant woman who has not yet given birth.

• My mother gave birth to me.
• My friend's mother adopted her when she was a baby.
• The pregnant woman is excited to become a mother when her baby is born.

The first two examples illustrate the traditional ways in which a woman can become a mother - through giving birth or adoption. The third example shows that a woman can also be considered a mother even before her child is born, as long as she is pregnant.