This very short excerpt from St. Zeno (translated by Thomas Livius in The Blessed Virgin in the Fathers of the First Six Centuries) talks about Jesus’ life in the womb of Mary. You can’t get more Advent-ish than that.
“…whilst preserving what He was, He meditated to be what He was not. Mingled thus with human flesh, He forms Himself an infant. The womb of Mary swells forth with pride….”