Why was circumcision performed on the 8th day of life?
The scriptures and modern science
An extract of an academic study
In the bible, circumcision was commanded on the eighth day of life and is found in Genesis 17:9-13 when God told Abraham to circumcise the males on the 8th day of life. 
Modern studies in haematology have shown that a key blood-clotting factor, Vitamin K, is not formed in human life until between the 5th to the 7th day (Holt and McIntosh, 1953:125-126). A second key clotting factor, Prothrombin develops to 30% of normal adult levels by the 3rd day of life and then peaks at 110% on the eighth day, before levelling off at 100% of a normal adult level (McMillen, 1984:93). More recently, neonatal platelet function has been shown to reach adult levels between the fifth and ninth day of life (Del Vecchio, Motta, and Romagnoli 2015:625). Thus for various converging reasons the eighth day of life is the earliest day for the optimal blood coagulation needed to limit excessive haemorrhage during circumcision.
This is a clear example of the Type 1 resonance where the designation of the eight-day for circumcision seems very unlikely to be arbitrary and the ancient practice has the additional benefit of thousands of years of actual experience observed by the participating rabbis.
This resonance is accepted as a well-winnowed  tradition by its practitioners (the rabbis) over many generations and indeed the practice continues to be observed in modern times. The writer of Genesis displayed a wisdom that was millennia ahead of modern science. This type 1 resonance is considered as positive due to the beneficial effects it has in the prevention of excessive haemorrhage during circumcision.
In addition to the blood coagulation resonances between Genesis 17:9-13 and circumcision on the eighth day of life, there is one further scientific resonance that is seldom discussed. The resonance concerns the maternal antibody levels in the neonate’s blood that are also close to maximum levels on the eighth day of life.
Maternal antibodies directed against common microorganisms are transferred via the placenta from mother to child thereby protecting neonates and infants during the time of full maturation of their immune system. The very large majority of these maternal antibodies belong to the IgG class of immunoglobulin, and these are preferentially transferred via the placenta up to the point of birth. These passively acquired antibodies enter the bloodstream of the neonate acting as a protective shield throughout the body in the same way as the actively produced antibodies beginning to develop in the neonate.  IgG maternal antibodies are known to be highly effective in protecting neonates and infants from most infectious agents like in the case of bacterial and certain viral infections for the first 6-12 month of life (Leuridan et al. 2011:2222 and Kilic et al. 2003:302). 
As we might expect, in time the transferred maternal antibody titres decline in the neonate’s blood. This due to the maternal antibodies being metabolized so eventually failing to protect the infant from infection. The infant’s own immune system should rapidly develop at this stage of life. It is plausible to suggest that the near maximal levels of maternal antibody in the neonate’s blood are in synergy with the optimal coagulation factors at the eight-day of life. This synergy means that not only will the circumcision be performed with minimal haemorrhage but also with a concomitant low risk of the common infections that can occur related to circumcision. This also is a clear example of the Type 1 resonance (though this time arguably in microbiology as opposed to haematology) where the designation of the eighth day for the circumcision again seems very unlikely to be arbitrary and again the ancient practice has the additional benefit of thousands of years of actual experience observed by the participating rabbis. This novel resonance deserves a mention in the long-standing tradition as practised by those involved in the rite.
The command to circumcise male infants on the 8th day of life seems to anticipate the findings of modern science by thousands of years. Surely the author of Genesis was divinely inspired?
 The biblical focus of brit milah (the covenant of circumcision) is the removal of that foreskin and the subsequent releasing of blood. Available at: http://www.ritualwell.org/ritual/blood-and-men [Accessed 30th June 2016].  See definition by John Bowker (1987:73).  Sometimes immunoglobulin isotype IgA antibodies contained in breast milk are also referred to as maternal antibodies.  Sometimes a bit longer according to Kilic et al. (p.303)  See the review of maternal antibody levels in the neonate by Stephan Niewiesk (2014 5:446).
This extract is from Charles Green- dissertation for the Master's degree , School of Divinity, University of Edinburgh (2016).
Full references can be obtained on request