Albert Barnes Commentary Leviticus 12:5

"But if she bear a maid-child, then she shall be unclean two weeks, as in her impurity; and she shall continue in the blood of [her] purifying threescore and six days." — Leviticus 12:5 (ASV)

Some have thought that this doubling of each of the two periods was intended to remind the people of the fact that woman represents the lower side of human nature and was the first to fall into temptation (1 Timothy 2:13–15; 1 Peter 3:7).

The ancients had a notion that the mother suffers for a longer time after the birth of a girl than after the birth of a boy. The period required for the restoration of her health in the one case was thirty days, and in the other, it was 40 or 42 days. This notion may have been connected with a general custom of observing the distinction as early as the time of Moses.