In many high income countries, it is standard practice to clamp the umbilical cord connecting mother and baby less than a minute after birth. However, clamping the cord too soon may reduce the amount of blood that passes from mother to baby via the placenta, affecting the baby's iron stores. On the other hand, delayed cord clamping, which is carried out more than a minute after birth, may also slightly increase the risk of jaundice. The World Health Organization now recommends cord clamping between one and three minutes after birth.

The researchers reviewed data from 15 trials involving a total of 3,911 women and their babies. They looked at outcomes for mothers and outcomes for babies separately, and looked at haemoglobin concentrations as an indicator of healthy blood and iron levels. While clamping the cord later made no difference to the risk of maternal haemorrhaging, blood loss or haemoglobin levels, babies were healthier in a number of respects. When cord clamping was delayed, babies had higher haemoglobin levels between one and two days after birth and were less likely to be iron-deficient three to six months after birth. Birth weight was also higher with delayed cord clamping.

"In light of growing evidence that delayed cord clamping increases early haemoglobin concentrations and iron stores in infants, a more liberal approach to delaying clamping of the umbilical cord in healthy babies appears to be warranted," said Philippa Middleton, one of the authors of the review based at the Australian Research Centre for Health of Women and Babies, Robinson Institute at the University of Adelaide in Adelaide, Australia.

Clamping the cord later did lead to a slightly higher number of babies needing treatment for jaundice, which is treated by light therapy. "The benefits of delayed cord clamping need to be weighed against the small additional risk of jaundice in newborns," said Middleton. "Later cord clamping to increase iron stores might be particularly beneficial in settings where severe anemia is common."