As researchers continue to learn more about autism—its causes and risk factors—it is increasingly clear that there isn’t a single driver for the developmental disorder, and no simple answer to why rates of the condition appear to be on the rise.
While much of the recent research has focused on genetic factors behind the range of autistic symptoms, researchers led by Keely Cheslack-Postava, a post doctoral research fellow at Columbia University, decided to turn their attention to a less-studied but still influential factor on a developing fetus—the mother’s womb. Specifically, the team focused on how the spacing between pregnancies, which influences the womb environment, can affect the development of autism.
Cheslack-Postava collected data on all California births of first- and second-born siblings from 1992 to 2002, and then calculated the time between the birth of the first sibling and the conception of the second. Among the more than 725,000 sibling pairs, those conceived less than 12 months after the birth of their older brother or sister showed a more than three-fold higher risk of having autism than those conceived more than three years after their sibling. (More on TIME.com: Study Linking Vaccine to Autism is “Fraudulent”)
“That there is a three-fold increase in risk of autism is obviously shocking, and dramatic,” says Dr. Andy Shih, vice president of scientific affairs for Autism Speaks. “But it is very plausible given what we know about reproductive physiology.”
The time between pregnancies is an important consideration for parents building a family, say experts, since gestation is known to deplete key nutrients, such as folate, which is important for normal fetal brain development. Telescoping the spacing between pregnancies has been linked to a higher risk of low birth weight babies as well as greater incidence of premature delivery. So it’s entirely plausible that the prenatal environment may play a role in autism risk as well.
But Cheslack-Postava is careful to note that the results do not definitively implicate closely spaced pregnancies as the cause of autism, and that the findings are not sufficient to advise women to space their pregnancies farther apart to decrease their child’s risk of developing the disorder. While it’s true that California, where the study’s data was collected, was among the first states to note a rise in autism rates several years ago, and while this trend correlates with the increase, from 11% to 18%, of births occurring within 24 months of a previous birth among mothers, there still isn’t enough evidence to confirm that pregnancy spacing is a major contributor to autism. Additional studies, including those that look more closely at other prenatal factors that might influence autism, are needed. “We need to look at other things that affect the quality of pregnancy and how they translate to a risk for autism,” says Shih. (More on TIME.com: Could Cell Phone Use During Pregnancy Affect Kids’ Behavior?)
In the meantime, women concerned about what impact their family planning decisions may have on the health of their future children should consult with their doctors about their individual risk-benefit profile. As more women decide to delay childbearing, for example, and start families at more advanced ages, their risk of birth complications or having children with birth defects increases the longer they wait between pregnancies, says Shih, so they need to weigh those considerations with newer data such as found in the current report. Cheslack-Postava agrees. “When it comes to beginning another pregnancy, women concerned about these results should discuss them with her doctor,” she says. “[the study] should be part of the discussion.”
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