Several:
1) Almost all of the medical benefits of circumcision (with the exception of decreased HIV transmission rate) are due to the fact that children under the age of 1 can't retract their foreskins to clean their glans penis. The increased rate of foreskin infections and urinary tract infections goes away after the foreskin can retract over the head of the penis, so if you wit to do it as an adult you lose almost all of the medical benefits.
2) The medical risks of adult circumcision are much greater. Its a sedated procedure so you have the risks of anesthesia, where newborn circumcision is done under local anesthesia (like when you get lidocaine at the dentist) which caries no significant risk. The infection and complication rates are also significantly higher.
3) The cost is drastically higher in an adult. You can circumcise a newborn for $250-500 as a part of their routine newborn hospital care, or in an outpatient Pediatric clinc. An adult circumcision means a multi-day stay in the hospital and an appointment in the OR with a urologist which will run you a conservative 10K, if not more.
You can decide if that is or is not enough of a difference to make it worth performing a circumcision on a child who cannot consent for themselves, but there definitely are good medical reasons to get it done early if you know you're going to do it.
That's a very American view. Of all the world's circumcised men, only about 10% were circumcised as infants. Other countries do it much later. If you want it done on the NHS in Scotland, you have to wait six months, and in Switzerland they make you wait two years.
I'm highly skeptical of the medical benefits of male circumcision anyway, as are most European medical organizations, but even if I were expecting a son and somehow knew for a fact that he would need to be circumcised at some point, I wouldn't have it done to him as a baby.
The only reason general anesthesia isn't used for neonates is because it's not safe. That alone would be an argument to wait.
Other reasons not to circumcise babies:
The foreskin has to be separated from the glans (the most painful part). If you wait, it separates naturally, usually by around puberty.
The post-operative pain will also be worse, and the wound will be in a diaper which is primarily there to contain urine and feces.
The risk of major injury or death is higher, since you're working with something smaller. The record payout for a botched circumcision is $22.8 million. It was said at the time that the victim "will never be able to function sexually as a normal male and will require extensive reconstructive surgery and psychological counseling as well as lifelong urological care and treatment by infectious disease specialists." Sure, cases like that are very rare, but why should they happen at all?
A newborn can die of blood loss after losing about three tablespoons of blood - way less than will be absorbed by a modern diaper.
There is a 5-10% risk of a revision (effectively a second circumcision) after neonatal circumcision, versus almost a zero rate for juvenile or adult circumcision. The revision rate is going up btw, though the reasons for that aren't clear.
Babies don't have much of an immune system. At least one baby died after circumcision in New York after being infected with the coldsore virus for instance, and another got brain damage.
It's almost exclusively males circumcised as infants that get meatal stenosis.
The child can't give informed consent, and some men out there who are seriously unhappy about having parts of their genitals cut off. It's *his* body, so why shouldn't it be his decision?
"For studies including boys born after 1995, there was a strong correlation between country-level (n=9) autism/ASD prevalence in males and a country's circumcision rate (r=0.98). A very similar pattern was seen among U.S. states and when comparing the 3 main racial/ethnic groups in the U.S."
Bauer, Kriebel, 2013
http://www.ncbi.nlm.nih.gov/pubmed/23656698
(the authors speculate that this may be due to paracetamol, but this seems unclear)
All of the really bad botched jobs I'm aware of were infant circumcisions. This might be partly because the people doing it often aren't surgeons:
http://journals.sfu.ca/cuaj/index.php/journal/article/view/200
"RESULTS: The overall response rate to the survey was 76%. From those, 85% were Family Doctors and Pediatricians, while the remaining 15% were Pediatric General Surgeons and Urologists. The circumcisions were carried out with the Gomco clamp and the plastibell at a rate of 62•5% and 37•5%, respectively. No surveyed-physician admitted to learning the procedure through a structured training course. 43% of the non-surgeons learned to perform a circumcision from a non-surgeon colleague. 31% of the participants were happy to perform a circumcision in a child born with a concealed penis, where circumcision is contraindicated.
With respect to the early complications post-circumcision, 100% of the surgeons vs. 63% of non-surgeons felt comfortable dealing with bleeding (p=0.046). 87.5% of the surgeons vs. 35% of non-surgeons were comfortable in dealing with urinary retention (p=0•01). 100% of surgeons vs. 53% of the non-surgeons were comfortable in dealing with a wound dehiscence (p=0•02). Moreover, 75% of the surgeons and ten percent of the non-surgeons were comfortable managing meatal stenosis (p<0•01). 62% of the surgeons vs. 36% of the non-surgeons were confident in dealing with a trapped penis post-circumcision (p=0•24).
CONCLUSIONS: Our survey findings indicate that the majority of physicians performing neonatal circumcisions in our community have received informal and unstructured training. This lack of formal instruction may help explain the complications and unsatisfactory results witnessed in our pediatric urology practice. Many practitioners do not seem to be aware of cases where neonatal circumcision is contraindicated and most non-surgeons performing this procedure are unable to look after common post-surgical complications."
My suspicion is that the push to circumcise early is largely because of the dollars involved. As one person put it, "it's like every baby boy is born with a coupon for $300 at the end of his penis, and the first person to cut it off gets the money".
