In ancient times the first born was sacrificed to the gods.
Where? When? This is an entirely inaccurate generalization.
The pagans in Canaan (and elsewhere in North Africa and the Levant) would sacrifice their children to Moloch. It wasn't just the first born.
The Achaean King Agamemnon got into trouble with his wife Clytemnestra, because he sacrificed his [grown] daughter Iphigenia at Aulis for favorable winds for his raiding party to Troy. (See Aeschylus' "Agamemnon" for the consequences of that decision, when Agamemnon returns home.)
As wicked as these ancient cultures may have been, the modern practice of the slaughter of the unborn is without peer and scale.