By Kathryn Doyle

(Reuters Health) - Waiting a few minutes to clamp the umbilical cord after birth is tied to better motor and social skills later in childhood - especially for boys, suggests a new study.

Delaying cord clamping is already known to benefit babies by increasing iron levels in their blood for the first few months of life, researchers write in JAMA Pediatrics May 26.

“There is quite a lot of brain development just after birth,” said lead author Dr. Ola Andersson of Uppsala University in Sweden. “Iron is needed for that process.”

For the new study, the researchers followed up on 263 Swedish children born at full term to healthy mothers about four years earlier.

As newborns, the children had been part of a larger study in which a total of 382 babies were randomly assigned to either early cord clamping (within 10 seconds of birth) or late cord clamping (at least three minutes after birth).

Four years later, the children were similarly intelligent regardless of when their cords had been clamped, but there were some notable differences.

“When you just meet a child, you wouldn’t see or notice any differences,” Andersson told Reuters Health. “But we could see the differences in fine motor function.”

A psychologist assessed the children using tests of IQ, motor skills and behavior. Parents reported child development in communication, problem solving and social skills.

Overall, brain development and behavior scores were similar for both groups of kids, and there was no difference in overall IQ scores.

But more children in the delayed cord clamping group had a mature pencil grip on the fine motor skills test and better skills on some social domains, compared to those whose cords were clamped early.

Divided by sex, the researchers only found noticeable differences in boys, not in girls.

Iron deficiency is much more common among male infants than among females, Andersson said.

“Girls have higher iron stores when they are born,” he said.

Delaying cord clamping by three minutes allows an extra 3.5 ounces of blood to transfuse to the baby, which is equivalent to a half a gallon of blood for an adult, Andersson said.

“There’s a lot of iron in that volume,” he said. “Even three minutes can have quite a lot of effect on the iron in the blood in the body for a long time after birth.”

The World Health Organization recommends waiting at least one minute after birth, or until visible pulsing stops, to clamp the umbilical cord. The latest American College of Obstetricians and Gynecologists opinion, from 2012, supports delayed cord clamping for premature infants, but says there is insufficient evidence to prove a benefit for full-term babies.

The new study provides evidence of benefit for full-term babies in a developed country where nutritional deficiency is extremely rare, Andersson said.

“When a baby transitions from inside the womb to outside the womb, if you think about what nature does, it is not to clamp the cord immediately,” said Dr. Heike Rabe of the Brighton and Sussex Medical School and University Hospitals in the UK.

About 60 years ago doctors started clamping the cord as early as possible, believing it may reduce the risk of hemorrhage for the mother, but we now know that is not the case, said Rabe, who coauthored an editorial published with the study.

Today, it is a big psychological hurdle for doctors to change how they have always done things, which in this case is to clamp the cord as soon as possible, she said.

Many mothers in developing countries are anemic, so babies are also anemic, Rabe said.

The new study found only a small difference at age four, but “if you did the same study in India you might see a marked difference, the reason is that there is excellent nutrition in Sweden,” she said.