Babies born by Caesarean section may have an increased risk of developing allergies, a new study suggests.

US scientists followed the progress over 1,200 newborns over a five-year period. The children were evaluated at four age intervals - one month, six months, 12 months and two years.

Other factors were also assessed, such as family history of allergies and asthma, exposure to tobacco smoke, household pets, illnesses and use of medication.

The study found that when exposed to high levels of common allergens, such as dust mites in the home and pets, children born by Caesarean section are five times more likely to develop allergies by the age of two compared to babies born naturally.

"This further advances the hygiene hypothesis that early childhood exposure to microorganisms affects the immune system's development and onset of allergies. We believe a baby's exposure to bacteria in the birth canal is a major influencer on their immune system," the scientists from the Henry Ford Hospital said.

Details of these findings were presented at the annual meeting of the American Academy of Allergy, Asthma and Immunology.