A study coming out in the respected journal Pediatrics reports that second children born soon after their older siblings are at a higher risk of developing autism, a developmental difference or disorder characterized in general by social and communication deficits. Note that even with the increased risk, the overall risk of autism even for these second siblings is still quite small.
The study is based on a large group derived from data maintained by the California Department of Developmental Services. Findings based on this database must include the caveat that anyone included in the database purposely sought and received services, so there can be a bias against and exclusion of people who sought services elsewhere or are living undiagnosed. It may represent about 75% to 80% of people in California with autism.
Closely spaced second borns had 3 times the autism risk
The authors assessed the rates of autism in second-born children relative to how long after the birth of their older sibling they arrived. Their analysis indicated that those second-borns who were conceived within a year of the older sibling’s birth were three times more likely to have autism. And they also identified an inverse linear relationship between time and autism -- the longer the time between the first birth and the conception of the second born, the lower the autism rate.
There was no assessment of what might cause this mathematical relationship, but the authors offered that the first pregnancy and birth might have depleted nutrients in the uterus, depriving the second-born of them. The authors comment that these findings are becoming increasingly relevant as the number of closely spaced births has grown from 11% of all births in 1995 to 18% in 2002.
Possible causes to explain the mathematical relationship: Are hormones implicated here?
The idea of nutrient depletion may be a plausible one, but another one comes to mind that the paper does not appear to address: The relationship between the sex of the previous occupant of the uterus and effects on the next occupant. A well-known hypothesis is that autism develops because of an excess of exposure to androgens in the womb, and research has shown that androgenization can occur in siblings that follow a male older sib in the womb. There’s also the “fraternal birth order effect,” in which the more older brothers a man has, the more likely that man is to be homosexual. This hypothesis, coupled with the idea that homosexual men are hyperandrogenized, has led to some speculation about the influence of an androgenized womb on those who experience that environment.
We have no way of knowing if there was any relationship between having a male older sibling and risk of autism in this cohort. Based on my review of the paper, the authors provide autism risk broken down by sex for the second siblings, but do not provide information about the sex of the first sibling or about the rate of autism in second siblings who followed a male older sibling. They likely have these data in hand and could perform this analysis; they may have done so and simply not reported it because there was nothing to report. But in my mind, in the context of the prevailing androgen hypothesis of autism, this analysis would be worth mentioning.
Not something to add to the worry list
Is this yet another thing for parents to worry about? The going wisdom on child bearing is that a mother should wait at least two -- if not three -- years between conceptions. Of course, some of us are older and don’t have the luxury of spacing our children that way. I, for instance, conceived our second son when his older brother was 8 months old and I was 33. They’re just over 16 months apart in age. Based on the risk categorization in this Pediatrics paper, our second son’s risk of autism from this factor alone would have been 3.3. The thing is, his older brother is the one who’s diagnosed with autism. I’ve spoken with other autism parents whose experience better fits the observations from the Pediatrics paper.
But these are all just anecdotes, not data. The best we can do is the best we can do. We can follow the going medical wisdom to space our children by two years at least -- or, we may be older and not have that luxury. At the very least, it’s not time to add sibling spacing to our list of autism causes. This paper simply showed a mathematical association, which does not mean a true, physiological connection, and their results require considerable further study.
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