Parents whose first child is a daughter with autism are more likely to have subsequent children with the condition, according to new research that assessed autism’s prevalence based on the health insurance records of more than 1.5 million families in the United States.

The study by a Harvard Medical School team is the largest ever to assess the potential for subsequent children in a family to develop autism after the diagnosis of a first child.

The new results are in line with earlier analyses that have shown that while boys more frequently are affected by autism, more genetic mutations are required for a girl to have the condition, suggesting an elevated overall risk for families with affected girls.

Based on the findings, from data on more than 39,000 children with autism, researchers found that when a firstborn daughter was diagnosed, autism occurred in 16.7 percent of younger brothers and 7.6 percent of younger sisters. When a firstborn boy was diagnosed, 12.9 percent of younger brothers and 4.2 percent of younger female siblings also developed autism.

“It is important to be able to provide worried parents who have one child with the condition some sense of what they can expect with their next child,” Nathan Palmer, the study’s lead author, said in a statement.

The findings in no way suggest subsequent children will have autism, even if an older sister is affected because the odds are still about 5 to 1 that a subsequent sibling will be unaffected, Palmer said.

On a practical level for doctors and genetic counselors, the team of researchers say the findings provide professionals with a new dimension on clarifying risk for families who already have one child with the disorder.

Autism is a neurodevelopmental condition that affects a child’s ability to communicate. Some children with autism do not speak. Many have difficulty with social interaction and some become obsessed with arcane interests, such as memorizing lists of numbers, for example, or collecting specific types of toys.

Early behavioral therapy helps children with communication and social skills.

Palmer and his colleagues reported their findings this week in JAMA Pediatrics.

Scientists at Long Island’s Cold Spring Harbor Laboratory were among the pioneers in autism research to identify a gender difference in autism. Boys, they found, are more likely than girls to have the condition in the first place.

Autism investigators at the lab have examined gender patterns and the genetics of the condition for more than a decade.

Researchers at the Centers for Disease Control and Prevention estimate that autism spectrum disorders affect 1 in 68 children in the United States regardless of gender, with boys having four times the risk of girls.

The new Harvard analysis quantified the risk for autism to occur more than once in a family based on the insurance records of families covered by Aetna from 2008 through 2016.

Among the families studied, there were more than 3.1 million children from age 4 through 18. About 39,000 of the children, or 1.2 percent, had received a diagnosis of autism. For every 100 boys with an older female sibling with autism, 17 received a diagnosis of autism or a related disorder.

Further parsing of the numbers, the researchers found that for every four boys with the condition, only one girl had it, reaffirming autism’s gender ratio.

“What we have provided here is context for families who already have children with autism or another similar disorder and need a clearer perspective on recurrence risk,” Palmer said.

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