shotgun marriage
A marriage that happens quickly due to an unplanned pregnancy. We knew it was a shotgun marriage when Frank's new wife had a baby five months after the wedding.
a ˌshotgun ˈwedding/ˈmarriage
(old-fashioned, informal) a marriage which takes place because the woman is pregnantThis expression probably refers to the father of a woman, who threatens to shoot the man unless he marries her.