New research from Kaiser Permanente has shown that younger siblings have higher chances of developing autism spectrum disorder (ASD) if they have an older sibling diagnosed with the disorder.

In a study published in the Journal of Developmental & Behavioral Pediatrics, the researchers showed that their findings were consistent whatever the gestational age was at the time of birth.

As a neurodevelopmental disorder, autism is characterized by impairments in communication and social interaction and repetitive and restricted behaviors. According to the Centers for Disease Control and Prevention, the condition is present in one in every 68 children.

It's still unclear what causes autism exactly but studies have been able to identify different environmental and genetic factors that play a role in the disorder's development. Earlier research from Kaiser Permanente, for instance, has shown that second-born children who were conceived sooner than two years or later than six years after their older sibling was born experience dramatically increased risks of ASD.

As for the current study, it was carried out with Kaiser Permanente members in Southern California, focusing on at least two siblings that were born to the same woman from 2001 to 2010 and at 28 to 42 weeks gestation period.

Over 53,000 cases were examined for the study, of which 592 had been diagnosed with ASD. The researchers found that those with older siblings with ASD had 11.3 percent ASD risk rates compared with the 0.92 percent risk that children with older siblings unaffected by the condition registered.

Additionally, gestational periods didn't make a difference. As long as children had older siblings diagnosed with the condition, they had higher risks of developing ASD. Younger boys with older brothers were also likelier to develop the condition compared with younger girls with older sisters.

"It's possible that parents who have an older child with an autism diagnosis are more likely to have their younger [children] tested too," said Darios Getahun, M.D., Ph.D., the study's senior author. He explained that this may be one of the reasons for the higher rate of diagnosis in younger siblings.

According to another research, vitamin levels during pregnancy may have a hand in upping autism risk in children, particularly folic acid, or vitamin B9, and vitamin B12. Keeping the mother in good health is necessary to ensure a child is in good health as well but it was found that women who have too much of the two vitamins in their body during their pregnancy run the risk of increasing the likelihood of their babies developing autism.

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