Michael Root:

If Jesus' humanity is full and uncompromised, one would think that he went through a normal development and thus was once an embryo. The church has assumed a full period of gestation, hence the dating of the Annuncation as March 25, a full nine months prior to Jesus' birth. A problem, of course, is that many of the traditional discussions of the incarnation assume pre-modern understandings of human conception.

While operating with such pre-modern understandings, however, Aquinas (Summa Theologiae, Third Part, Question 31, Article 5) seems clearly to indicate that the conception of Christ in the womb of Mary includes the formation of a body that is itself simply natural (although produced supernatually). I see no reason to deny, and every reason to affirm, that Jesus was an embryo.

Charlotte Allen:

All human beings start out as embryos. Jesus was/is fully human as well as fully divine. Therefore, Jesus started out as an embryo. QED, no?