"The Eternal Being, who knows everything and who created the whole universe, became not only a man but (before that) a baby, and before that a foetus inside a Woman's body."

Lewis may have had in mind Philippians 2:5-8 (ESV): "Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped,but emptied himself, by taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross."