It depends what is meant by “fully human” in your question.
If you are asking “Was Adam ‘fully human’ in the ontological sense”, then my answer is yes.
If you are asking “Did Adam experience all that we humans today experience” then no he did not.
Was Adam born via a womb? No. Was he ever a fertilized egg? No. Etc.
And now you might think of applying the same notions to the Advent of Jesus, God with us. If you are open to applying your won reasoning, you might find that Jesus can be fully human without ever being a ‘fertilized egg’ (more accurately a fertilized oocyte; humans are not reptiles or chickens). Without using a sex cell from Mary, Mary could be a surrogate for gestating and bearing Jesus, without violating her vows/betrothal to Joseph. Think about it ...
And; I have it from a good source; he had no belly button!