Researchers at Henry Ford Hospital in Detroit followed 1,187 newborns and measured levels of the antibody immunoglobulin E (IgE) in blood samples collected from the babies at birth, 6 months, 1 year and 2 years.
IgE is associated with the development of allergies and asthma; higher levels indicate increased risk.
The study found that IgE levels during infancy were 28 percent lower in children whose mothers were exposed to indoor pets during pregnancy (indoor prenatal pet exposure) compared to babies from pet-free homes.
IgE levels were 16 percent lower in infants who had indoor prenatal pet exposure and were born vaginally compared to infants who had indoor prenatal pet exposure and were delivered by cesarean section.
IgE levels were 33 percent lower in infants of European, Asian or Middle Eastern descent who had indoor prenatal pet exposure, compared to 10 percent lower in black infants with indoor prenatal pet exposure.
The study was published online Aug. 8 in the Journal of Allergy and Clinical Immunology.
"We believe having a broad, diverse exposure to a wide array of microbacteria at home and during the birthing process influences the development of a child's immune system," senior study author Christine Cole Johnson, chair of the Department of Public Health Sciences, said in a hospital news release.
The finding supports what's known as the hygiene hypothesis, a theory that early childhood exposure to infectious agents affects immune system development and the risk of allergies and asthma, she added.