I recently saw [Bill Maher's documentary](http://www.imdb.com/title/tt0815241/) and he mentions that the virgin birth is only mentioned in two of the gospels. He goes on to bug Christian people several times by asking questions such as "How can you believe in the virgin birth if it's only mentioned in two of the gospels?". I know for a fact that the virgin birth is mentioned in the gospels of **Matthew** (1:18) and **Luke** (1:26-35), however, it is mentioned in *neither Mark nor John*. In spite of that, millions of Christians world-wide take the virgin birth for granted. My question is, as stated above, why isn't the virgin birth of Jesus mentioned in all four gospels?