Giving many courses of steroids to women at risk of giving birth prematurely increases the risk of having a smaller-sized baby, a Canadian-led international study suggests.

A single course of corticosteroids has been shown to reduce the risk of death in newborns, respiratory distress and bleeding in the brain of infants born to women at risk of giving birth before 37 weeks of pregnancy.

But doctors have moved beyond what the evidence supported and repeated courses of the prenatal drugs — also known as antenatal — has become common, according to a commentary that accompanies the study in the Dec. 20 issue of the medical journal The Lancet.

In the largest trial to date, Dr. Kellie Murphy of the department of obstetrics and gynecology at Toronto's Mount Sinai and her colleagues analyzed data on more than 1,800 women at risk of preterm birth who received one course of prenatal corticosteroids 14 to 21 days before the study began.

Of these participants, 937 were randomly assigned to take multiple courses of corticosteroids, and 921 to placebo, every 14 days until week 33 or delivery, whichever came first.

The researchers found that infants who had multiple courses of the drugs had similar levels of death and disease as those given sugar pills. But compared with the infants receiving placebos, the corticosteroid group:

• Weighed less at birth (2,216 g vs. 2,330 g).
• Were shorter (44.5 cm vs. 45.4 cm).
• Had a smaller head circumference (31.1 cm vs. 31.7 cm).

"[I]n women who remain at increased risk of preterm birth after receiving an initial course of antenatal corticosteroids, multiple courses every 14 days are not recommended," the study's authors concluded.

Follow-up studies on the infants should look for any changes in behaviour, growth, glucose tolerance and blood pressure that other studies have suggested could be problems, the Australian commentators said.

"In the meantime, the evidence suggests it is prudent for obstetricians not to prescribe repeat injections of antenatal corticosteroids," wrote Prof. John Newnham of the School of Women's and Infants' Health at the University of Western Australia in Perth and Prof. Karen Simmer, of the Neonatal Care Unit at King Edward and Princess Margaret Hospitals, also in Perth.

"Single-course therapy is of considerable benefit, but we should be aware of the potential dangers of giving too much of a good thing."

Dr. Amos Grunebaum, director of obstetrics at Cornell University in New York City, agreed, adding that he worries some women who need the one shot may now be reluctant to get it.

"The concern is that some women might misread this and say: 'I don't want it at all.' That's not at all what this paper says," said Grunebaum, who was not involved in the study.

For women who received multiple shots of corticosteroids during their pregnancy and are worried about whether the medication had an impact on their children's health, they should talk to their pediatricians, he suggested.

With files from the Canadian Press