Leviticus 12:1-8 describes the purification system for women that give birth in ancient Israel. This is a part of the ritual purity laws and gives instructions on how a new mother is to be purified of niddah (womb bleeding) uncleanness after childbirth. One puzzling aspect of childbirth purification is why must the mother wait only 40 days after giving birth to a boy, but 80 days for a girl? Also, she has to give a burnt, and sin offering even though that is typically for people who were recently healed of a bodily discharge disorder caused by sin (Lev 15:13-15;28-30). Having a child isn’t a sin or a disorder so why?
Science says that a woman’s womb takes around 6 weeks (42 days to heal). If a woman gives birth to a son, the mother is unclean for seven days just like in menstruation. Then she must wait for her womb to heal (be purified) from bleeding for 33 days, making a total of 40 days. It’s two weeks and 66 days (80 days) for a baby girl. One theory is that purification for girls is twice as long because it represents the fact that there are two womb carriers (or two females). Another theory is it was related to the circumcision of the males. Maybe somehow the male is purified by the circumcision so the timeframe is longer for girls. However, this would mean all uncircumcised boys would require 80 days and I don’t see any reference to that in the text. The other option is that since a baby girl has a womb and while it may not be bleeding, this extra time for purification represents her ability to give birth in the future. I lean towards the second one since it seems more consistent. There is a resource down below that goes further into this explanation of Leviticus chapters 12-15.
When the 40 or 80 days are over, the mother brings two offerings (sin and burnt offering) to the Tabernacle. Childbirth isn’t a sin, but we are all born with sin nature. It seems more likely that sin offering covers the sin nature of the child rather than any sin of the mother. It was believed by the rabbis at that time a person can be guilty of and even punished for sin from the womb. That is why the disciples asked Jesus if the blind man who was born blind because of sins he committed before he was born or his parent’s sins in John ch. 9. Some people will take this verse out of context to suggest that Jews in the 1st century believed in Reincarnation but that is not the case, they were talking about sin in the womb. More on Reincarnation in the bible here.
This is why Jesus refers to salvation as being born again (John 3:1-15). We are all born in water in our natural birth, but the re-birth in the holy spirit (John 3:5-7) which is connected to the blood sacrifice of the messiah (Jesus) makes us born again when we believe in him. In Luke 2:21-24, baby Jesus is circumcised 8 days after he was born, and then 40 days after his birth Mary and Joseph gave a purification offering according to the law of Moses in Leviticus 12. This shows that Jesus’ parents helped Jesus fulfilled the law even as a child.