The more children a woman has, the less likely she is to suffer from allergic rhinitis or allergic conjunctivitis, a study from Italy suggests.

A small family size has previously been reported as a risk factor for allergies in children, giving rise to the “hygiene hypothesis” that suggests a reduction in infections in childhood leads to increasing allergic sensitization.

To look into the relationship between family size and mothers’ allergies, Dr. Francesco Forastiere in Rome and colleagues surveyed 1755 nonsmoking women ages 35 to 74 living in four areas of northern central Italy.

The investigators reviewed self-reports of allergy symptoms and medical records, and report their findings in the medical journal Allergy.

The rates of allergic rhinitis fell from 30 percent among women with no children or one child, to 16 percent among those with four or more children, the team found. Results were similar for allergic conjunctivitis, declining from 36 percent to 17 percent.

Forastiere’s group suggests several theories that could explain their findings.

For example, a woman’s immune response could be related to fertility, or pregnancy may prevent allergies by altering the balance of inflammatory and anti-inflammatory factors, or perhaps through “changes in maternal nutrition” with increasing numbers of children, they suggest.

Another possibility is that the hygiene hypothesis may apply to mothers as well as children.

SOURCE: Allergy, April 2005.