Sociologist Peter Bearman, and team set out to find out whether there might be a link between the length of time between the birth of one child and his/her brother or sister and autism risk. They found that in cases where pregnancies were less than 12 months apart, the risk of autism in the second-born child was three times as high, compared to pregnancies spaced at least three years apart.
They also found that pregnancy spaced between 1 to 2 years apart had double the risk of autism in the second child compared to those at least 3 years apart.
The researchers examined data from the California Department of Developmental Services to determine how many children had been diagnosed with autism.
Even when other factors that might influence autism risk were taken into account, such as the age of the mother or father, low birth weight, or being born preterm, "we see this really profound association". The authors added that they could not clearly determine what the causes might be.
Peter Bearman said:
"When you see something so robust and so stable, it provides an important clue as to what we should be looking at next."
They suggest that possibly a mother who soon becomes pregnant again may not have fully replenished crucial nutrients. Perhaps parents are better at identifying autism-like traits, such as delayed milestones, after their second child is born.
The authors explained that their study did not include autism in first-born children.
Previous studies had found a link between higher autism risk in a second child if the first child had an autism spectrum disorder, including Asperger's syndrome.
The authors concluded in the journal's abstract:
"These results suggest that children born after shorter intervals between pregnancies are at increased risk of developing autism; the highest risk was associated with pregnancies spaced <1 year apart."
According to data from the CDC (Centers for Disease Control and Prevention), the incidence of autism in the USA has risen tenfold during the last four decades, to approximately 1 in every 110 children in 2006.
Although increased awareness and better diagnosing techniques account for some of the increase, Bearman believes that other factors have also had an impact.
A comprehensive study published in the BMJ (British Medical Journal) last week clearly showed that a 1998 report by Dr. Andrew Wakefield linking childhood vaccines to autism risk was "an elaborate fraud". BMJ editor in Chief, Dr. Fiona Godlee said "The MMR scare was based not on bad science but on a deliberate fraud.. (such) clear evidence of falsification of data should now close the door on this damaging vaccine scare." Link to article about the report
"Closely Spaced Pregnancies Are Associated With Increased Odds of Autism in California Sibling Births"
Keely Cheslack-Postava, PhD, MSPH, Kayuet Liu, DPhil, Peter S. Bearman, PhD
PEDIATRICS January 10, 2012 (doi:10.1542/peds.2010-2371)