That's important to note in how the Jews considered life...it was always at conception. Not just a clump of cells.
As far as I know, this is simply his assertion. I'm not aware of any reason to think it's true.
Matthew:
“Look, the virgin shall conceive and bear a son,
and they shall name him Emmanuel,”
which means, “God is with us.”
When Joseph awoke from sleep, he did as the angel of the Lord commanded him; he took her as his wife, but had no marital relations with her until she had borne a son; and he named him Jesus.
There's no reference here to Jesus before his birth. There's conception, there's birth. Nothing between on which to base any conclusions.
The language in Luke is similar. Conception and birth. No statement about status. Mary's pregnancy is referred to as "expecting a child."
He'd have a better case to make if he referred to John the Baptist, who is said to have leaped in the womb. Personally I think that's non-literal, but at least there'd be some basis. But the ethical question behind abortion simply hadn't come up. Knowledge about development, spontaneous abortions, questions about when you consider a person brain-dead, you're not going to find these in the Bible. Trying to get answers from Biblical accounts about other things is lousy exegesis.