The Eighth Day:
Why is the circumcision preformed specifically on the 8th day? Scientific research has proven that on the 8th day of a baby’s life his blood clots at its fastest rate. Until the 8th and from the 9th the clotting is slower, rendering the 8th day the best possible day in a human being’s life to circumcise. Some claim that this is why Hashem commanded us to perform the circumcision on this day, in order to preserve the baby’s health in the best way possible; however can we indeed claim that this is the REASON we circumcise on the 8th day, or the reverse true? Hashem ensured that the clotting reach its peak on this day, because this was the day we would have to circumcise? If so, we are back to square one - what is the reason which binds the circumcision specifically to the 8th day of a baby’s life?
http://aliyosshmuel.com/thinkjewish.asp?AID=197
Interesting! Under what circumstances would the mother not be considered impure?