Matt 1:19-20

Then Joseph her husband, being a just man, and not willing to make her a publick example, was minded to put her away privily. But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost.

KJV

Joseph was going to end the betrothal to Mary when he found out she was pregnant.  He didn't realize that she was still a virgin and the baby she was carrying was conceived by the Holy Spirit.  He didn't rush right into his plans though.  He thought on these things.  While he thought an angel appeared and gave him more information.  This stopped Joseph from carrying out his plans.  This is a great lesson.  We must not be hasty with our 'righteous' fixes for situations.  We might not have ALL of the information.  It was key that Joseph be the one who was Mary's husband.  It was Joseph who was from the line of David resulting in Mary and Joseph going to Bethlehem when the census was ordered.  This fulfilled the prophecies that stated the Messiah was going to be born in Bethlehem.  Joseph had a key role in the birth of the Messiah.  It was a good thing that Joseph seriously weighed his actions before doing them, allowing him to receive further clarification and direction from God!  Let us follow his example and don't be hasty to fix things until we hear from the Lord!