Yes, Jesus was the God-man while in the womb of Mary. In Hebrews 10:5, a passage that refers to Christ, we read, “So when he came into the world, he said, “Sacrifice and offering you did not desire, but a body you prepared for me.” The human body begins in the womb. This also fits with the gospel accounts which attribute Mary’s conception and pregnancy to the miraculous power of the Holy Spirit. That which was in Mary’s womb was spoken of as the “holy offspring” who “shall be called the Son of God.” While the Greek text of this verse, Luke 1:35, is difficult to translate, it literally says, “that which is begotten or conceived, is holy, he shall be called the Son of God.” Matthew adds to this truth when he wrote, “Joseph, son of David, do not be afraid to take Mary as your wife, because the child conceived in her is from the Holy Spirit” (Matt. 1:20). This is immediately followed by the words, “She will give birth to a son and you will name him Jesus, because he will save his people from their sins” (1:21). The child in the womb of Mary was called the Son of God. He would be true humanity and undiminished deity united together in one person by the miracle of the virgin birth. Without the virgin birth He would have a sinful nature like us and would not have been qualified to become our substitute for sin. As true humanity, he could be our representative; as sinless humanity, he was qualified to die for our sins. Without the conception in the womb, he would not have been true humanity.
There was a Gnostic heresy not too many years after the birth of Christ that taught that Christ only came on the person of Jesus after his birth. Such a Jesus could not save us and would be no different from any other so-called religious leader who had sins of his own.