Autism was not even on my family’s radar when Joe came along. What went wrong? Why did he develop autism when his brothers and cousins did not? It’s a question I’m sure all parents in a similar situation have asked themselves.
Despite the somewhat controversial conclusions of a recent twin study, it’s pretty clear that ASDs are strongly genetic in origin. However, the genetics are complex (involving multiple genes) and many experts believe that environmental factors-defined as anything that affects development from the time of fertilisation-may interact with genetic factors to increase the risk.
A recent meta-analysis of 40 studies, published in the July 11 issues of the journal Pediatrics, looked for evidence of specific perinatal (relating to the period shortly before and after birth) and neonatal (relating to the first 28 days of an infant’s life) risk factors for autism.
As we explained in the Australian Autism Handbook, meta-analyses allow researchers to pool together data from several trials and analyse the results. The larger patient numbers means we are sometimes able to pick up treatment effects (or in this particular example risk factors) that were not evident in the individual small trials.
Conversely, in some small trials a risk factor may be falsely identified, simply due to random chance. If we combine the results of several studies the chance of this happening is reduced. However, when performing meta-analyses it’s essential that the individual trials are similar enough in design to be suitable for pooling; if not, the results will be meaningless.
The recent meta-analysis by a group of US researchers found the following perinatal and neonatal factors were all associated with a statistically significant increased risk of autism:
- foetal distress
- birth injury or trauma
- multiple births
- maternal bleeding during labour and delivery
- summer birth
- low birth-weight
- congenital malformations (birth defects)
- low 5-minute Apgar score
- feeding difficulties
- meconium aspiration, and
- neonatal anaemia or jaundice.
On the contrary, factors found not to be significantly associated with autism risk included:
- anaesthesia use
- assisted vaginal delivery
- post-term birth
- high birth weight, and
- head circumference (other studies have found that rapid head growth associated with autism occurs after birth).
It’s important to note that a risk factor is simply something that increases your chance of developing a disease or a condition. It’s obvious that not all low birth-weight babies—or children who experience birth complications—will go on to develop autism. (And if by chance you’re having a new baby this summer the odds are that he/she will be perfectly fine!) To provide some examples from the meta-analysis: the investigators found that a baby weighing less than 1.5 kg at birth was three times more likely to develop autism than a baby weighing in the normal range at birth. The relative risk of a twin developing autism was 1.77—that is, not quite double that of a singleton child. The highest risk identified was for babies who asphyxiated meconium (a birth complication associated with foetal distress); their risk of developing autism was increased over seven-fold.*
The same investigators published an earlier meta-analysis looking at prenatal (occurring or existing before birth) risk factors for autism. In that study they identified advanced maternal and paternal age, maternal bleeding, maternal diabetes during pregnancy, being first born and maternal prenatal medication use as risk factors.
This raises the chicken and egg question with the current study—did something go wrong with baby earlier, during foetal development, which resulted in a complicated delivery, or did the complicated delivery (potentially causing oxygen deprivation) trigger autism in the child, perhaps because they were already genetically vulnerable?
The answers to these questions remain elusive, but each new study improves our understanding of this perplexing condition.
*If you need reminding what the normal odds of having a child with ASD are it’s anything from 1:160, based on Australian research, to 1 in 110 – or approximately 1% – according to the lastest US data.
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That chicken and egg question is huge isn’t it? And I am sure when we find out more on these issues that there might be some chickens and some eggs, as it were.
Great to point out that environmental factors are anything post-conception; I’d generally not think of them in that way but can see it’s very much the case.
Thanks B for taking what is very complex and explaining it simply and clearly.