Pope Shenouda III
The sentence   “before they came  together, she was found with child”
  St Luke the Evangelist aims at proving that  Christ was
 conceived of a virgin who did not know any man, for two reasons:
(a) The fact that  Christ was not born  through the natural way from two parents as all people are, but was born  of a virgin, is proof of His Divinity; proof that He was born of the  Holy Spirit as expressed by the angel: "...for that which is  conceived in her is of the Holy Spirit" (Matt. 1: 20).   (b) His birth from a virgin convinces us  that He did not inherit  Adam's sin. Thus He would be able to save us  since the One without sin is able to die for the sinners.
   Therefore the Apostle concentrated on  the Virgin not knowing any man before giving birth to    Christ in order to  prove the Lord's Virgin Birth. But the fact that the   Virgin Mary did  not know any man after giving birth to    Christ is a self-evident truth.