From CNA:
The phrase "conceived and bore" is used repeatedly (see Gen. 4:1, 17) and the individual has the same identity before as after birth. The same word is used for the child before and after birth ("Brephos", that is, "infant" is used in Lk. 1:41 and Lk. 18:15.)
God knows the pre-born child. "You knit me in my mother's womb . . . nor was my frame unknown to you when I was made in secret" (Ps. 139: 13,15). God also helps and calls the pre-born child. "You have been my guide since I was first formed . . . from my mother's womb you are my God" (Ps.22:11-12). "God . . . from my mother's womb had set me apart and called me through his grace" (St. Paul to the Galatians 1:15).
Scripture repeatedly condemns the killing of the innocent.
This flows from everything that has been seen so far. God's own finger writes in stone the commandment, "Thou shalt not kill," (Ex.20:13, Deut. 5:17) and Christ re-affirms it (Mt. 19:18 — notice that He mentions this commandment first.) The Book of Revelation affirms that murderers cannot enter the Kingdom of heaven (Rev. 22:15).
The killing of children is especially condemned by God through the prophets. In the land God gave his people to occupy, foreign nations had the custom of sacrificing some of their children in fire. God told His people that they were not to share in this sin. They did, however, as Psalm 106 relates: "They mingled with the nations and learned their works . . . They sacrificed their sons and their daughters to demons, and they shed innocent blood, the blood of their sons and their daughters, whom they sacrificed to the idols of Canaan, desecrating the land with bloodshed" (Ps. 106: 35, 37-38). This sin of child-sacrifice, in fact, is mentioned as one of the major reasons that the Kingdom of Israel was destroyed by the Assyrians and the people were taken into exile. "They mutilated their sons and daughters by fire . . . till the Lord, in his great anger against Israel, put them away out of his sight" (2 Kg. 17: 17-18). Not even for "religious freedom" can the killing of children be tolerated.