Why is a woman declared unclean twice as long after giving birth to a girl as opposed to a boy?

Leviticus 12:1-5. First, let’s look at why the woman would be considered unclean after giving birth. According to the text, bleeding made the woman unclean. Some suggest this regulation protected women from infection. Others see it as a symbol for a lack of wholeness. Perhaps because blood meant life, bleeding implied the opposite, symbolizing death and sin.

Now, with regard to the question above, we don’t really know why God gave this command. Sometimes God’s instructions reflected cultural views held during that time. Ancient peoples may have believed a woman bled longer after delivering a girl. Or perhaps the passage acknowledges the higher value their culture placed on males. Though such a perspective seems unfair today, Biblical teaching raised the status and rights of women far above any other laws or cultures of the time.

–The Quest Study Bible