Jason Goroncy offers some helpful words about this subject, which puzzles many:
The miracle of the virgin conception is a judgement against the possibility of the creature producing its own word of revelation and reconciliation. It is a judgement against us thinking that we can know God apart from God’s initiative, and that we might save ourselves apart from God’s bloody intrusion into our situation. It is the proclamation of God’s gracious and free decision to be God for us, to unveil for us, to reconcile us. And it is the proclamation of God’s gracious and free decision to save us, and that by becoming personally involved – literally enfleshed – in the deepest depths of creaturely experience. This is why it is Good News. In PT Forsyth’s words, ‘The Virgin birth is not a necessity created by the integrity and infallibility of the Bible; it is a necessity created (if at all) by the solidarity of the Gospel, and by the requirements of grace’. (Positive Preaching and the Modern Mind, 14).
As others have pointed out, both Forsyth (pre-Barth) and Goroncy (post-Barth) sound a lot like Karl Barth himself on this subject.