And on the eighth day the flesh of the boy’s foreskin is to be circumcised.
The law here pronounces women lying-in ceremonially unclean. The Jews say, "The law extended even to an abortion, if the child was so formed as that the sex was distinguishable." 1. There was some time of strict separation immediately after the birth, which continued seven days for a son and fourteen for a daughter, Lev 12:2, Lev 12:5.
Commenting on Leviticus 12:1-5
And in the eighth day the flesh of his foreskin shall be circumcised. Or the foreskin of his flesh, that is, of the man child born according to the law, Gen 17:12 and this seems to furnish out a reason why a male child was not circumcised before the eighth day, and why it was then, because before that its mother was in her separation...
And in the eighth day - Before this time the child could scarcely be considered as having strength sufficient to bear the operation; after this time it was not necessary to delay it, as the child was not considered to be in covenant with God, and consequently not under the especial protection of the Divine providence and grace, till this rite had been performed.