Matthew begins the story of Jesus' life with Jesus' miraculous conception in this verse. In the beginning, Joseph did not understand that Mary's pregnancy was the result of the Holy Spirit's work. As a result, he was faced with the difficult decision of deciding what to do about their upcoming wedding. In that situation, it is reasonable and common sense to conclude that Mary had been unfaithful. The full truth is only revealed to Joseph when he is informed by an angelic messenger.
As you can imagine, Joseph's heart was broken when he discovered his beautiful bride was pregnant after an extended visit to her cousin, Elizabeth. It is clear that the child she was carrying was not his own biological Son since we read that the couple had not yet come together and consummated their marriage.
This mysterious conception was made possible by the Holy Spirit, the third person of the eternal Godhead. Mary was to be filled with the Holy Spirit, and the power of the Highest would surround her so that the child she bore would be holy and called Immanuel - 'God with us.' The Lord Jesus was not conceived of flesh nor by human will. In fact, He was the only begotten Son of the Father.

Dear Heavenly Father, Your love for us is so great that You sent Your only begotten Son to be born of a virgin to become our Savior through His vicarious death and resurrection. Thank You for allowing Him to die and rise for me so that I could have His resurrected life in me. It is a blessing to have the Bible and to know the truth it contains. I pray that my relationship with You will grow stronger and more grace-filled with each passing day. 
In Jesus's name I pray. Amen.