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In Scientific Facts in the Bible: Amazing Truths Written Thousands of Years Before Man Discovered Them, pop-apologist Ray Comfort quotes from Genesis 17:12 (NKJV) and writes,
Why was circumcision to be carried out on the eighth day? Medical science has only recently discovered that blood-clotting in a newborn reaches its peak on the eighth day, then drops. This is the day that the coagulating factor in the blood, called prothrombin, is the highest.
Is this why circumcision was to be done on the eighth day following birth? Or is there are more mundane explanation rooted in the world of the biblical author?
CIRCUMCISED THE EIGHTH DAY
When my son was born, he was almost immediately given a shot of vitamin K. The reason for this is that virtually all infants come into the world with deficient levels of vitamin K. Because vitamin K is an important component of blood clotting, a shot of it decreases the likelihood that an infant could bleed out. The Centers for Disease Control notes that vitamin K deficiency bleeding (VKDB) is a risk for babies from birth to even past the six-week mark. Given that VKDB has a 20% mortality rate among those who have not received the shot, getting it to an infant as soon as possible can be lifesaving.
My son received vitamin K minutes after birth and was circumcised just two days later with no issues, no doubt thanks in part to the shot. In fact, there is evidence to show an 82% decrease in the risk of bleeding following a circumcision in infants who received vitamin K versus those who do not. Given that this practice of bolstering infants’ clotting mechanisms didn’t begin until the 1960s, the risks to babies who were circumcised before that time would have been greater. And since VKDB is not merely a risk for infants in the first few days of life, even circumcising on the eighth day would have posed a risk in antiquity. To be on the safe side, wouldn’t it have been wiser of God to command that sons be circumcised closer to their first year of age? Why then the eighth day?
In Genesis 17, Yahweh appears to Abram and instructs him that the covenant between the deity and Abram as well as his offspring entails the rite of circumcision. “You shall circumcise the flesh of your foreskins, and it shall be a sign of the covenant between me and you” (v. 11). This act of circumcision, per divine command, is to be done when a male is eight days old (v. 12). The text never explains why the sign is to be administered when the infant is eight days old as opposed to the first day or at a year old. And unfortunately for Abram, his thirteen-year-old son Ishmael, and his male slaves, the rite of circumcision was given well after the eight-day mark (vv. 23-27). (It is easy to imagine that the pain a ninety-nine-year-old man might experience having his foreskin removed is of a different quality than that of an eight-day-old.) As noted above, Ray Comfort thinks the ritual was to be done at eight days because that is when the ability of a newborn’s body to clot reaches its peak. But is this the most parsimonious explanation?
The rite of circumcision was not unique to Israelites, and it certainly did not originate with them. In his book Circumcision: A History of the World’s Most Controversial Surgery, historian David Gollaher observes that the earliest extant evidence of circumcision comes from the middle of the third millennium BCE in ancient Egypt. On the wall of a tomb, one finds “a well-preserved bas-relief of temple priests in the act of cutting the genitals of two young noblemen.” Inscriptions on the wall give the impression that the surgery could lead to intense pain such that the recently circumcised might pass out. “The Egyptian ritual must have presented an opportunity for a youth, on the threshold of manhood, to demonstrate his mastery over bodily pain,” Gollaher opines. Indeed, among cultures that practiced it, circumcision was often tied to puberty or marriage. What set the Israelite practice apart was that instead of being performed on adolescents or adult males, it was administered on sons that were but eight days old. Consequently, circumcision was no longer tied to puberty or marriage. But this doesn’t mean that it was entirely untethered. Within the corpus of the biblical texts, there are some hints as to why the rite was given to infants on the eighth day following birth.
In Leviticus 12, Yahweh tells Moses that when a woman gives birth to a son, she is unclean for seven days (v. 2). The seven-day formula is connected explicitly to menstruation which, per Leviticus 15:19, results in a seven-day period of uncleanness. If the woman is unclean for seven days, naturally this would mean that she is clean on the eighth. Her renewed status coincides with the timing of her son’s circumcision which, Leviticus 12:3 tells us, is to happen on the eighth day following birth. The question then is, Why the eighth day? The answer is that in all likelihood the child was considered unclean. In Leviticus 15, it is explicit that anyone who comes into contact with what a menstruating woman has touched is unclean. Moreover, a man that has a sexual encounter with a menstruating woman that results in him coming into contact with “her impurity,” that is her, her menstrual flow, is also unclean for seven days (v. 24). Given the explicit connection between childbirth and menstruation and that the child would have necessarily come into contact with his mother’s sexual organs and blood, it stands to reason that the text of Leviticus 12 is implying the newborn son was also unclean. It is only when this ritual impurity begins to subside on the eighth day that the male infant is given the sign of Israel’s covenant with Yahweh. Shaye Cohen notes that the connection between circumcision and impurity not only existed in other cultures but that the removal of the foreskin signifies some kind of purified status. This is almost certainly implied by Yahweh’s command to Abram in Genesis 17 for he warns the patriarch in v. 14 that “[a]ny uncircumcised male who is not circumcised in the flesh of his foreskin shall be cut off from his people; he has broken my covenant.”
Comfort’s definitionally eisegetical attempts at understanding circumcision on the eighth day demonstrates his ignorance of the Bible, his disrespect of it, or both. A likely explanation for circumcision on the eighth day following birth is readily available simply by reading these texts in their own context.
 Ray Comfort, Scientific Facts in the Bible: Amazing Truths Written Thousands of Years Before Man Discovered Them (Bellflower, CA: Living Waters Publications, 2016), 13-14.
 See Centers for Disease Control, “Frequently Asked Questions (FAQ’s): Vitamin K and the Vitamin K Shot Given at Birth” (last reviewed 1.1.21), cdc.gov.
 Rebecca Decker, “Evidence on: The Vitamin K Shot in Newborns” (updated on 4.9.19), evidencebasedbirth.com.
 Or, were he an intelligent designer, why not allow infants to be born with a sufficient level of vitamin K?
 David L. Gollaher, Circumcision: A History of the World’s Most Controversial Surgery (New York: Basic Books, 2000),1.
 Gollaher, Circumcision, 2.
 Nahum Sarna, The JPS Torah Commentary: Genesis (Philadelphia: The Jewish Publication Society, 1989), 125.
 Matthew Thiessen, Jesus and the Forces of Death: The Gospels’ Portrayal of Ritual Impurity Within First-Century Judaism (Grand Rapids, MI: Baker Academic, 2020), 33.
 Shaye J.D. Cohen, Why Aren’t Jewish Women Circumcised? Gender and Covenant in Judaism (Berkeley, CA: University of California Press, 2005), 19.