I. John the Baptist and Jesus Christ in the Womb
There's no question from Scripture that the Child in Mary's Womb is very much alive right from the start. When Mary visits Her cousin Elizabeth (Luke 1:39-45), John the Baptist "leaps for joy" in his mother Elizabeth's womb when Elizabeth heard Mary's voice. That is, the still-unborn John the Baptist recognized the presence of God in Mary's Womb. No question that Luke depicts both John and Jesus as very much alive. And at this time, Jesus is less than one month old. We know this because John is six months older than Jesus (Luke 1:26), Mary went to visit "at that time," right after the angel announced to Her (Luke 1:39), and because Mary stayed three months, and left right before the birth of John (John 1:56-57).
So Elizabeth was about six months pregnant with John when the Visitation occurred, while Mary was less than a month pregnant with Jesus. And yet both Jesus and John are treated as fully alive, human beings with intrinsic dignity. More than that, Jesus is fully God Himself. Now, this plainly establishes that from a Christian perspective, unborn children are children from the start, and children with intrinsic dignity and worth.
II. Jesus, Mary, and Abortion
There's another way of looking at this, which I concede is unpleasant for any of us who love the Blessed Virgin Mary. Imagine that Mary declined to become the Mother of God in Luke 1:26-38 (it wouldn't have happened, for obvious reasons, but bear with me). This refusal to become pregnant by declining to conceive might have been a sin, but it wouldn't have killed Jesus Christ (obviously). On the other hand, to have "ended the pregnancy," that is, had Mary had an abortion, She would have killed Jesus. Once She conceives Christ in Her Womb through the power of the Holy Spirit (Luke 1:35), He's there, not simply some genetic material that may someday be formed into Christ. And that's why there's a large emphasis placed on conception as well as birth (see Luke 1:31, Luke 2:21, Matthew 1:23, etc.).
III. The Bible Teaches that Life Begins at Conception
It's not just here, it's throughout the Bible. We're told that original sin haunts us from our conception in our mother's wombs (Psalm 51:5), while David proclaims in Psalm 22:10, "From birth I was cast on you; from my mother’s womb you have been my God." Psalm 139:10 proclaims to God, "For you created my inmost being; you knit me together in my mother’s womb," a passage only more beautiful now that we know about DNA. God tells us that He formed us in our mothers' wombs in places like Isaiah 44:24, Isaiah 44:2, and most famously, Jeremiah 1:5. Isaiah himself was formed to be a prophet in the womb of his mother (Isaiah 49:5). Although Hosea 2:5 is a metaphoric prophesy, it still speaks of life beginning at conception (same with Isaiah 33:11).
In Genesis, Rebekah is troubled because while pregnant with Esau and Jacob, "the babies jostled each other within her," which God explained was because they were predestined to be rivals, saying that the "two peoples from within you will be separated" (Genesis 25:22-23). Jacob grabs his brother's heel while still in the womb (Hosea 12:3; Genesis 25:24-26). Samson is dedicated to God from his mother's womb (Judges 13:7; Judges 16:17).. Job, in his misery, wishes he'd died in vitro (Job 10:18-19), which only makes sense if he was alive. Jeremiah says something very similar in Jeremiah 20:17-18. Paul is called to be an Apostle before birth (Galatians 1:15). And of course, in Luke 1:44, Elizabeth calls the unborn John the Baptist her "baby." And on and on it goes.
When the Saints in the Bible, as well as the Prophets, the Angels, and even God Himself describe life as beginning at conception, and the conceived child as a person -- a person who God already has a plan for, and who already has a personality -- the dispute is settled. To reject the life and dignity of the unborn child, you have to reject the word of God. You also have to go against literally thousands years of Judeo-Christian teaching on the subject. Simply read the condemnations of abortion in the Didache, or even in apocryphal literature like the Apocalypse of Peter, and you'll quickly see that both the early Christians and the early heretics realized that the unborn child was just that: a child. To toss off the explicit word of God and a constant line of Tradition is to simply cease to be Christian. So you can't be Christian and pro-choice. You must choose. Choose life.
This day I call the heavens and the earth as witnesses against you that I have set before you life and death, blessings and curses. Now choose life, so that you and your children may live.