Anyway on AO3, I remember reading this story where evil scientists stole Steve's sperm. The Avengers had to go get it back.
When they got there, however, they found that the bad guys had used Steve's sperm to fertilize Natasha's eggs and a baby (a girl I believe) was born. So they take the kid back home.
I remember in one scene Steve had to give the baby to Tony to watch because she spat up on him. So he went to go get changed. Tony was terrified with the kid. And then I think Natasha was terrified of leaving Tony alone with the kid so she came running out in a towel to keep on them?