Natasha can't have kids of her own - she had that option taken from her by force at the Red Room, and never really got over that.
When Steve assigns her to train Wanda Maximoff, Natasha rather expected to dislike the girl, especially following the events from Age of Ultron.
But when she goes to officially introduce herself to the little witch, Wanda is an absolute mess. She's curled in a corner of her assigned room, sobbing her heart out over her dead twin, and her powers are going haywire. She's terrified, and hurting, and all alone in the world...and the second that Natasha feels that protective urge start up, she knows that this girl is absolutely going to be her daughter.
Natasha accidentally adopts Wanda
When Steve assigns her to train Wanda Maximoff, Natasha rather expected to dislike the girl, especially following the events from Age of Ultron.
But when she goes to officially introduce herself to the little witch, Wanda is an absolute mess. She's curled in a corner of her assigned room, sobbing her heart out over her dead twin, and her powers are going haywire. She's terrified, and hurting, and all alone in the world...and the second that Natasha feels that protective urge start up, she knows that this girl is absolutely going to be her daughter.