Did God place a living fetus in the womb of Mary who then became Jesus? No, God did not place a living fetus in the womb of Mary. We do not know exactly what happened at Jesus’ conception, but Mary is literally and genetically the mother of Jesus; at least, his physical form. If God did place a viable fetus in the womb of Mary, not using her egg, then that would deny Old Testament prophecies that the Messiah would be born to the lineage of David (John 7:42; Psalm 132:11; Jeremiah 23:5). Mary was a descendent of David and the mother of Jesus (Luke 3:23-38 ). If a viable fetus was simply placed in Mary’s womb, then it could have been any woman who could have given birth to him. This would mean there was no lineage to Jesus’ birth, and that would violate the Scripture.

This would further mean that Jesus was not really Jewish. He would be a Gentile with no real Jewish lineage. But he was a Jew, made under the law (Galatians 4:4). He had to be circumcised, which is a requirement for Jews. He also participated in the Passover meal which is for the Jews. And, the woman at the well specifically called Jesus a Jew. She said, “… How is it that You, being a Jew, ask me for a drink since I am a Samaritan woman?” (John 4:9). Jesus did not correct her by saying he was not Jewish. Why? Because he was Jewish.

Jewishness was traced to the mother. Please consider these quotes.

• “Jewishness is not in our DNA. It is in our soul. The reason it is passed down through the maternal line is not just because it is easier to identify who your mother is. It is because the soul identity is more directly shaped by the mother than the father.”  http://www.chabad.org/theJewishWoman/article_cdo/aid/968282/jewish/Why-Is-Jewishness-Passed-Down-Through-the-Mother.htm
• “According to halakha, to determine a person’s Jewish status (Hebrew: yuhasin) one needs to consider the status of both parents. If both parents are Jewish, their child will also be considered Jewish, and the child takes the status of the father (e.g., as a kohen). If either parent is subject to a genealogical disability (e.g., is a mamzer) then the child is also subject to that disability. If one of the parents is not Jewish, the rule is that the child takes the status of the mother (Kiddushin 66b, Shulchan Aruch, EH 4:19)…All branches of Orthodox Judaism and Conservative Judaism today maintain that the halakhic rules (i.e. matrilineal descent) are valid and binding.”  http://en.wikipedia.org/wiki/Who_is_a_Jew%3F
• “A person’s status as a Jew is completely dependant on the mother.” http://www.jewishanswers.org/ask-the-rabbi-category/the-basics-of-judaism/?p=933
• “The original and current Jewish definition of a born Jew is someone whose mother is Jewish. Even though the Torah forbids a Jewish woman to marry a Gentile man, if she does, her children will still be Jewish.” http://www.beingjewish.com/identity/whoisajew.html

So, if God placed a viable fetus in the womb of Mary, then that would violate Scripture, and it would also mean that any woman could have been the mother of Jesus, even one outside the prophecies of God concerning the Messianic lineage.