In the case of Christ He started out as a fetus in the womb (just like we all do). His body developed in Mary.
Jesus was conceived by a woman through the Holy Spirit .... because He needed to become human. He needed to go through the same developing process to be born a human.
Jesus in the womb developed just like we all do ... the only difference being He was supernaturally conceived through the Holy Spirit.
In Luke 1.31–3,
Gabriel addresses Mary. 'Do not be afraid, Mary', he says, 'for you have found favour with God. ' 22 He continues: 'And behold, you will conceive in your womb and bear a son, and you will name him Jesus.
so are you saying Jesus was not a human being until He left the womb?
No, He was a human being in the making ... it's the process God put in place for humans to reproduce .... a physical process of reproducing. This process allows for a body to develop.
Looking at the context of
Hebrews 10:5 it becomes clear that the body prepared for Jesus is prepared for sacrifice on the cross. And like Exodus where the nailing of the slave’s ear to the wooden doorpost is the act of an adult, not connected with the slave’s birth, so Christ’s preparation for the nailing of his body to the cross is the act of an adult, not connected with Christ’s birth.
5 Consequently, when Christ came into the world, he said,
“Sacrifices and offerings you have not desired,
but
a body have you prepared for me;
6 in burnt offerings and sin offerings
you have taken no pleasure.
7 Then I said, ‘Behold, I have come to do your will, O God,
as it is written of me in the scroll of the book.’”
8 When he said above, “You have neither desired nor taken pleasure in sacrifices and offerings and buernt offerings and sin offerings” (these are offered according to the law), 9 then he added, “Behold, I have come to do your will.” He does away with the first in order to establish the second. 10 And by that will we have been sanctified through the
offering of the body of Jesus Christ once for all.
Christ “coming into the world” of course includes his birth as well (see
Gal. 4:4.
2 Co.5:21etc.) but it is also adult Jesus coming into the world of the Jews.
The writer of Hebrews’ own explanation “through the
offering of the body of Jesus Christ” is that this passage “but
a body have you prepared for me;” is about the offering of the body of the adult Jesus, for whom his whole life, not just his birth, was a preparation.