Adam Clarke on Revelation 12:2

And she being with child cried, travailing in birth, etc. - This, when taken in connection with the following verses, is a striking figure of the great persecution which the Church of Christ should suffer under the heathen Roman emperors, but more especially of that long and most dreadful one under Diocletian. The woman is represented as Being with child, to show that the time would speedily arrive when God's patient forbearance with the heathen would be terminated, and that a deliverer should arise in the Christian world who would execute the Divine vengeance upon paganism.