A new meta-analysis of studies strengthens the conclusion that autism risk is lower when pregnancies are spaced a year or more apart. The findings are in line with broader research showing that short birth intervals pose a number of medical risks for both mothers and babies born too closely behind an older sibling.
The new report, by researchers with the World Health Organization and US Office of Population and Reproductive Health, appears today in Pediatrics. Its analysis encompasses more than a million children in previously published studies on pregnancy interval and autism risk.
The analysis associates subsequent pregnancy intervals of three to four years with nearly half the autism risk as pregnancies spaced closer than 12 months. The analysis also suggests a modestly increased risk when one pregnancy follows another by less than two years. At the same time, the analysis found that the overall risk of autism remained relatively low - generally under one percent - even among children born from pregnancies that began less than a year after a previous one.
What’s behind the link?
The studies did not show why short pregnancy intervals may predispose to autism. But the authors discuss some possibilities:
* The link between short pregnancy intervals and autism may be related to nutritional factors such as folate insufficiency. Folate (vitamin B-9) is crucial for healthy prenatal brain development. But a woman’s folate levels can remain low for up to 12 months after a pregnancy. In 2013, a landmark study found that a prenatal supplement containing folic acid (the synthetic form of folate) can significantly reduce the risk of autism if taken in the weeks before and after conception (regardless of pregnancy spacing).
* Another factor may be inflammation, which can remain elevated in the mother’s body for 9 to 10 weeks after she gives birth. A number of studies have suggested that maternal inflammation during pregnancy can affect brain development in ways that predispose to autism.
“At this point, all we can say with confidence is that healthy pregnancies are of the highest importance,” comments developmental pediatrician Paul Wang, Autism Speaks head of medical research. “Women who are pregnant or thinking of having a baby should consult with their doctors to help promote optimal health in areas such as nutrition, weight, management of any chronic conditions and being up to date on appropriate immunizations.”
The analysis produced a number of additional findings:
* The association between short pregnancy interval and autism was strongest for the subtype of autism formerly called “autistic disorder.” This subtype is generally placed on the more-severe end of the autism spectrum and often associated with intellectual disability and seizures.
* By contrast, there was no strong association between short pregnancy interval and the autism subtypes formerly known as Asperger syndrome and PDD-NOS (pervasive developmental disorder not otherwise specified). These two subtypes generally fall on the “milder” end of the autism spectrum and tend to be associated with normal to high intelligence.
* Three of the autism studies in the analysis looked at pregnancy intervals longer than five years. They found a small but significant increase in risk. More research is needed to verify and understand this association, the new report concludes.
* Analysis of three separate studies on pregnancy spacing and developmental delay likewise found increased risk with shorter pregnancy intervals. The risk dropped with each additional month between pregnancies up to 5 years.
* Analysis of two studies on pregnancy interval and cerebral palsy found increased risk with very short pregnancy intervals of under 6 months.
For past coverage of related research, also see: “Autism Risk Lowest when Pregnancies Spaced 2 to 5 Years Apart.”
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