קדש לי כל בכור פטר כל רחם
Chazal (Kiddushin 29b) derive from here that the term b’chor, firstborn, applies only to the firstborn of the mother. A firstborn who is the first for the father, but not the mother, does not become consecrated b’kedushas bechorah, the sanctity of the firstborn. We wonder why this is so? The bechorim are sanctified due to the miracle concerning their salvation when all the heathen firstborn of Egypt were slain. The Jewish firstborn were spared. Regarding the Egyptian firstborn, no distinction was made whether it was the father’s or mother’s firstborn – they all died. Indeed, if no firstborn was in…