However, emergency Caesareans increase the risk of a child developing asthma later to 60 percent. Children born vaginally but with assistance from vacuums or forceps have a 20-percent increased risk, the study said.

The study, published in the Journal of Pediatrics, is based on data from 1.7 million births registered at the Medical Birth Registry at the Norwegian Institute of Public Health.

"We found a moderately strong association between birth by Cesarean section and asthma in childhood," Dr. Mette Christophersen Tollanes of the Norwegian Institute of Public Health and the University of Bergen, said in a statement.

Tollanes says there are two theories -- Caesarean-born babies' immune systems may be affected by not being exposed to their mothers' bacteria during birth, or babies born by Caesarean section are less exposed to stress hormones and compression of the chest that empty the baby's lungs of amniotic fluid.

However, Tollanes points out, neither theory accounts for the difference in asthma risk between emergency and planned Caesarean sections.