Mark's Jesus eschews signs, so it makes sense that Mark omits any mention of Jesus' nativity such as Matthew and Luke have relying on Isaiah

 

 The virgin birth, according to Isaiah, is a sign, after all.

Therefore the Lord himself shall give you a sign [σημεῖον -- LXX]; Behold, a virgin [παρθένος -- LXX] shall conceive, and bear a son, and shall call his name Immanuel. 

-- Isaiah 7:14

Behold, a virgin [παρθένος] shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.  

-- Matthew 1:23

For unto you is born this day in the city of David a Saviour, which is Christ the Lord. And this shall be a sign [σημεῖον] unto you; Ye shall find the babe wrapped in swaddling clothes, lying in a manger.

-- Luke 2:11f.

There shall no sign [σημεῖον] be given unto this generation.

-- Mark 8:12