“Even babies who are well cared for and have access to good medical care, and whose family has access to enough food, can still become iron deficient,” Dr. Mascola said.

Delayed cutting may offer additional benefits beyond reducing the likelihood that a child will develop iron deficiency. In addition to iron, umbilical cord blood also contains immunoglobulins and stem cells, and researchers speculate that delayed cord clamping might aid immune function and tissue healing, though this has not been well studied. “It’s hard to measure those things, but there’s every reason to believe that there are benefits there as well,” Dr. Mascola said.

In preterm babies, delayed cord clamping has additional benefits, including a lower incidence of intraventricular hemorrhage, or bleeding in the brain, and a serious bowel infection called necrotizing enterocolitis. Both are common complications of premature birth.

While many hospitals have adopted delayed cord clamping for preterm infants, it isn’t known how many are doing it routinely for full-term babies. “Many, many places are beginning to adopt this into their setting, so it may take some time, but I think the practice is probably changing,” said Dr. Tonse Raju, chief of the pregnancy and perinatology branch of the National Institutes of Health.

The A.C.O.G. statement cautions that babies with delayed cord clamping may be more likely to require treatment for newborn jaundice, a buildup of blood components that causes yellowing of the eyes and skin, though Dr. Raju said this concern is based on older studies that may have been flawed.

If the mother or baby needs urgent medical attention, clamping the cord immediately may also be necessary, the group says. One such scenario is when a baby doesn’t start breathing right away and might need to be moved immediately to a warming table with the necessary medical equipment.

Once babies start to breathe, they shift from receiving oxygen from the placenta to getting it through the lungs. “You hear that loud, beautiful cry, and what they’re doing is opening up the vasculature as well as the airways of their lungs and making room for that extra blood,” Dr. Kattwinkel said. He and other researchers believe that it may be beneficial to leave the cord intact — with oxygenated blood still flowing from the placenta — at least until a baby takes a few breaths.