Parents of children with autism spectrum disorder (ASD) appear to curtail attempts to have more children.
The research*, from the USA, found that parents who have a child with ASD are about one third less likely to have more children than families without an affected child.
For the first few years after the birth of a child with ASD, parents' reproductive behaviour was similar to that of the control families. But birth rates differed in subsequent years with families whose first child had ASD having a second child at a rate of 0.668 that of control families. Women who changed partners had a slightly stronger curtailment in reproduction with a relative rate of 0.553 for a second child.
"These results are, to our knowledge, the first to quantify reproductive stoppage in families affected by ASD by using a large, population-based sample of California families,” state the authors.
The findings, which appear in JAMA Psychiatry, stem from the largest study of its kind on further child-bearing after a child has been diagnosed with the disorder.
These are the first data to indicate that this is a reproductive decision.
"While it has been postulated that parents who have a child with ASD may be reluctant to have more children, this is first time that anyone has analysed the question with hard numbers," said Professor Neil Risch, director of the University of California, San Francisco (UCSF) Institute for Human Genetics.
Most previous research into the heredity of autism has ignored a possible decision on the part of parents with affected children to reduce their subsequent child-bearing, a situation that occurs with some birth defects and has been termed "reproductive stoppage." As a result, previous estimates of the odds of having a second child with the disorder may have made the risk appear lower than it actually is.
Using California state health records, the authors identified 19,710 families in which a child with ASD was born between 1990 and 2003. The families were matched with 36,215 control families without an ASD-affected child.
Overall, families whose first child had ASD were one-third less likely to have a second child than control families. Families in which a later-born child was the first to be affected by ASD were equally less likely to have more children.
Researchers said that subsequent childbearing appeared to be normal until the time an affected child would typically start to show symptoms or be diagnosed, indicating that the stoppage was likely the result of a parental choice or circumstance, rather than a reproductive problem.
According to Risch, the findings do more than simply change the picture of risk for having another autistic child. "Our work shows that not only do people with ASD have fewer children than others," he said, "but in families where a child has ASD, the fact that the parents choose to have fewer children means the genes that predispose to ASD are less likely to be passed on to future generations."
This, in turn, leads to a paradox that has yet to be solved, he said.
"ASD has an important genetic component, which should be diminishing over time due to this reduction in childbearing," Risch said. "Yet over the past several decades, the incidence of ASD has risen dramatically."
* Hoffmann T J, et al. Evidence of Reproductive Stoppage in Families With Autism Spectrum Disorder. A Large, Population-Based Cohort Study. JAMA Psychiatry. Published online June 18, 2014. doi:10.1001/jamapsychiatry.2014.420