WASHINGTON (Reuters) - US researchers have linked mothers' infection during pregnancy to asthma, the most common chronic disease among American children, in their offspring.

A 16-year study following nearly 400,000 births in California found that when mothers had an inflammation known as chorioamnionitis and if a baby was born pre-term, that child was more likely to develop asthma by age eight.

Such inflammation of the placenta or amniotic fluid can result from a number of bacterial infections of the vagina, including E. coli and group B streptococci.

Chorioamnionitis complicates eight percent of pregnancies, according to the study published on Monday in the Archives of Pediatrics and Adolescent Medicine.

Dr. Darious Getahun of the West Los Angeles Medical Center, who led the study, said doctors had assumed that being born pre-term was the reason children developed asthma later in life.

The study showed that chorioamnionitis is a factor in asthma independent of pre-term birth, Getahun said in a telephone interview.

Getahun said the findings point out the importance of prenatal care.

"Women sometimes tend to underestimate the importance of prenatal care and miss opportunities for finding this type of problem before it occurs," he said.

In 2006, nearly 10 million US children were diagnosed with asthma and 6.8 million had an asthmatic episode, making it the most common chronic childhood disease, according the US Department of Health and Human Services.

The researchers used electronic health records to follow 397,852 single births from 1991 to 2007 at Kaiser Permanente medical centres in Southern California.

Blacks are about 25 percent more likely to have asthma than whites and this infection could explain much of the difference, the researchers said.

If a mother had the infection and if her baby was born pre-term, the risk of asthma was 98 percent higher for black children, 70 percent higher for Hispanic children and 66 percent higher for whites, the researchers found.