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Wondering when to have baby number 2? You may want to consider your timing carefully. A new study published in the journal Pediatrics shows that closely spaced pregnancies may raise the risk of autism in the second-born child. The highest risk was found in babies conceived within a year of their older siblings birth — in this case the younger children are three times as likely to be diagnosed with autism compared to siblings spaced more than three years apart. Autism risk was also heightened, but to a lesser degree, for siblings spaced 12 to 23 months and 24 to 35 months apart.

In this study, Columbia University researchers examined birth records and autism diagnoses in more than a half-million California children from 1992 to 2002. Other risk factors were also taken into account, including the age of the parents.

While babies conceived between 1 and 12 months of their older sibling’s birth had three times the risk of autism, babies conceived within 12 and 23 months had almost twice the risk of autism, and babies conceived between 24 and 35 months had a 26 percent greater chance of having autism. Third-born siblings spaced less than three years apart also saw a heightened autism risk

So should you freak out if you’re expecting a baby spaced within these ranges? It’s probably too early to start planning pregnancies based on this one study according to its authors. However it is important food for thought.

“We think this is important research because the finding provides an important clue about what may possibly be modifiable risk factors for autism,” study author Keely Cheslack-Postava, a postdoctoral research scholar at Columbia University in New York City told HealthDay. “But because we don’t know what it is about the inter-pregnancy interval that actually matters we can’t at this point say that increasing space between pregnancies is necessarily advice that should be given.”

The exact cause of the heightened risk of autism in closely spaced pregnancies is still unknown. However, researchers speculate that perhaps the mother’s body hasn’t had time to recover completely from the first pregnancy, replensihing critical nutrients such as iron and folate. Parents may also be on the lookout for developmental delays the second time around as well.

According to the Centers for Disease Control and Prevention, almost 1 out of every 11o kids in the U.S. is diagnosed with an autism spectrum disorder. And with the recent news revealing that the study linking autism to vaccines was a fraud, the debate over the true causes of autism continues — and there are a lot of parents out there looking for answers.