Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine. ~ Exodus 13:2
The phrase “openeth the womb” is a reference to conception and not of birth.
In Genesis 29:31-32, the Lord opened Leah’s womb, and then she conceived. In Genesis 30:22-23 God opened Rachel’s womb, and then she conceived.
In the book of Judges we see this truth played out in the life of Samson. Samson’s mother was barren and the angel of the Lord appeared to her saying the following:
Behold now, thou art barren, and bearest not: but thou shalt conceive, and bear a son. Now therefore beware, I pray thee, and drink not wine nor strong drink, and eat not any unclean thing: For, lo, thou shalt conceive, and bear a son; and no razor shall come on his head: for the child shall be a Nazarite unto God from the womb: and he shall begin to deliver Israel out of the hand of the Philistines. ~ Judges 13:3b-5
Verse 7 tells us that Samson would be a Nazarite to God from the womb to the day of his death. In Judges 16:17 Samson says, “I have been a Nazarite unto God from my mother’s womb…”
Throughout the history of Israel, from Moses until the time of Christ, there were millions of children who were firstborns, and everyone one of them were to be sanctified to the Lord in the womb.
When we come to the New Testament, we see this same truth in the life of John the Baptist, who was was filled with the Holy Spirit from his mother’s womb.
When Mary, the mother of Jesus, shared the good news with Elizabeth that she was going to have a son, the Bible tells us the babe leaped in Elizabeth’s womb and she too was filled with the Holy Spirit.
And it came to pass, that, when Elisabeth heard the salutation of Mary, the babe leaped in her womb; and Elisabeth was filled with the Holy Ghost: And she spake out with a loud voice, and said, Blessed art thou among women, and blessed is the fruit of thy womb. And whence is this to me, that the mother of my Lord should come to me? For, lo, as soon as the voice of thy salutation sounded in mine ears, the babe leaped in my womb for joy. ~ Luke 1:41-44