NEW YORK - Pinching off the umbilical cord as soon as possible after birth may not be best for babies, according to a new review.
Studies showed infants tended to have higher levels of iron in their blood when the cord was clamped up to three minutes after birth, rather than the usual less than one minute.
An extra minute or two might allow more iron-rich blood to flow from the placenta to the baby, researchers said.
Doctors started clamping the cord immediately after birth in the 1950s and 1960s as one of a number of steps to try to lower the mother's risk of bleeding, according to lead author Susan McDonald.
"It is an important finding particularly for women giving birth in circumstances where anaemia or poor nutrition is common," McDonald, a professor of midwifery at La Trobe University in Melbourne, Australia, told Reuters Health.
"Adequate iron levels are important for brain growth and overall development in the infant and up to two years of age."
McDonald and her coauthors reviewed the results of 15 trials including a total of 3,911 women and their full-term babies, who were randomly chosen to have their umbilical cords clamped early or late.
Clamping the cord later made no difference to women's risk of hemorrhage, amount of blood loss or levels of hemoglobin - iron-containing molecules in red blood cells that carry oxygen from the lungs to the rest of the body.
However, babies had higher hemoglobin levels one to two days after birth and were less than half as likely to be iron deficient three to six months later with delayed clamping.
Birth weight was also about 100 grams - or 3.5 ounces - higher, on average, when the cord was clamped later.
Researchers do not know yet how much of a difference these small variations make to the overall health of babies, according to Deb Erickson-Owens, who studies birth practices and umbilical cord management at the University of Rhode Island in Kingston.
Clamping later was also linked to a small increase in the risk of jaundice, according to the review from the Cochrane Collaboration, an international research organisation that evaluates medical evidence.
About 4.4 per cent of babies in late-clamping groups developed the condition, compared to 2.7 per cent in early-clamping groups.
Jaundice is a yellowing of the skin and eyes due to high levels of bilirubin, a yellow substance in hemoglobin. It is common in infants a few days old and often goes away on its own.
Many factors influence the risk of jaundice - not only delayed cord clamping, Erickson-Owens said. When the condition is poorly managed it can be dangerous, but that is rare in the US, she added.
She believes the potential benefit of increased iron levels outweighs the risk of jaundice, but questions remain.
"Iron is an important mineral especially in early brain development but we don't know if these better iron stores translate to a better brain," she said. "Studies are currently ongoing looking at this and will follow the babies out to two years of age."
The results of the review are important news for doctors as they stay abreast of changing birth practices, but mothers also have a say in how early the cord is clamped, Erickson-Owens said.
"Mothers deserve to have the benefit of birth care practices that are supported by the best available research evidence," she said. "Many mothers are not aware of these practices but they should be."