"Long ago, before we were born, when we were inside our mothers' bodies, we passed through various stages."

Lewis is referring to the now-debunked theory of recapitulation, which is often summarized as "ontogeny [the origin and development of an individual organism] recapitulates phylogeny [the sequence of events involved in the organism's evolutionary history]." Based on a series of drawings created by German biologist Ernst Haeckel (1834-1919), it has a long history of controversy, including an article by Waldo Shumway of the University of Illinois Department of Biology published in The Quarterly Review of Biology in 1932, years before Lewis wrote Mere Christianity. Here is a short article from Science magazine, 1997, discussing the problems with Haeckel's theory.