Once a routine procedure for newborn boys, circumcision is falling rapidly out of favor in the United States – even as growing evidence suggests that the surgery may reduce the transmission of HIV and other sexually transmitted diseases.

In recent years, many doctors and medical groups, including the influential American Academy of Pediatrics, have stopped recommending routine circumcisions because they believed there wasn’t enough evidence that it’s medically necessary.

More states also have been cutting Medicaid funding for circumcisions, which typically pays for about a third of all circumcisions each year. Sixteen states no longer cover the surgery. A number of private insurers also have stopped paying for the procedure.

As a result, overall circumcision rates in the U.S. have fallen to the lowest level in more than half a century, from 63.5 percent in 1993 to 55.9 percent in 2003, the last year for which federal data are available.

Removing the foreskin from the penis can’t prevent transmission of sexually transmitted diseases. The best prevention techniques remain reducing the number of sexual partners and using condoms.

But as new HIV cases remain at a stubborn 40,000 a year and infections continue to disproportionately affect minorities and the poor, some prevention experts again have begun urging doctors to recommend circumcision for newborns. A small number are even advising uncircumcised adult men at high-risk for HIV to undergo the procedure.

In the first random trial on the topic, published this month in the Public Library of Science Medicine, researchers tracked for nearly two years more than 3,200 men in South Africa who were randomly assigned either to be circumcised or not. Only 20 men in the first group became infected, whereas 49 in the second did. Researchers halted the experiment early because the results were so convincing.

Other, smaller studies done over the last several years strongly suggest that circumcision also protects against gonorrhea and syphilis. In 2003, a review of 2,000 men who visited an inner-city STD clinic during a one-year period found that uncircumcised men were as much as twice as likely to have either or both infections.

“Other than condoms and antiretroviral drugs that block transmission during childbirth, circumcision is the next best tool we have to (slow) infections,” said Dr. Thomas Coates, a professor of infectious diseases at the University of California, Los Angeles’ David Geffen School of Medicine and a well-known HIV prevention expert.