“Speak to the children of Israel, saying, If a woman conceive seed, and bear a man-child, then she shall be unclean seven days, as in the days of the impurity of her sickness shall she be unclean.”
Firstly it is emphasised that the woman who gave birth was to be seen as unclean ‘for seven days', as she was in the case of menstruation (the days of her impurity - see Leviticus 15:19, another case where a sacrifice was also required). After all similar blood flows came from her in both cases. The flow of blood was a constant reminder of the woman's mortality. It also rendered her untouchable at the time, especially by men.
Whether it was seen as a reminder of prospective death, only averted by the later intended sacrifice, or whether rather it was seen as indicating that the woman was in an ‘imperfect' and life diminishing state, and therefore at the time a blemished state, is something that cannot be demonstrated. But clearly she was seen as at that time ‘not her whole self', and in no condition to approach God. Through childbirth she was undergoing the consequences of the fall afresh. She was unclean.
So a divinely perfect period, seven days (or for a girl twice seven days), the number of days connected with creation, was to be allowed for her first recovery. It was a period of severe uncleanness. She was enduring all the consequences of the fall. The number seven was a number used of divinely perfect and completed activity, and ‘seven days' was the period of creation, Thus it may here have been seen as being in order that God might do His re-creating work in restoring her. Or it may simply be because seven was for all nations seen as a divine number of completeness. And it was after all in a sense already prescribed for in the covenant of circumcision (Genesis 17:10-14). It fitted in with circumcising a boy child on the eighth day.
This period then emphasised man's fallen state. During this period of serious uncleanness the woman would be left relatively alone, helped only by those women (such as her mother) who were prepared to become unclean by helping her. And the child too would be unclean, if only because of contact with its mother. But at the end of the seven days, in the case of a boy, the severe uncleanness would be seen as at an end, to be followed on the eighth day with a ceremony in which blood was spilt, and in which the child was welcomed into the people of God. Hopefully by this stage the blood flow would have ceased, to be followed by the continuing discharge of lockia which would not be seen as outwardly as serious, and therefore was seen as occurring in a period of lesser uncleanness.