Autism is an extremely common condition. The initial risk of having an autistic child is about 1.5%, and one in 68 children will be diagnosed with this diagnosis. Autism is more common in boys than girls, with a 5:1 male to female predominance. Twenty years ago, the causes of autism were not well understood, and a damaging article in the Lancet suggested that autism was caused by childhood vaccinations. Fortunately, this article has since been discredited, and scientists now understand the causes of autism much better.
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Currently, researchers agree that autism is a multifactorial disorder, resulting from a combination of genetic and environmental risk factors, with particular emphasis on the former. Evidence for the genetic basis of autism is strong and long-standing. Approximately 40% of children diagnosed with autism have identifiable genetic mutations. Since a multitude of genes control brain development, many different genetic alterations can cause autism. Examples of genetic mutations associated with autism include 11q23 and 19q13 deletions, ANKRD11, STXBP1, DYRK1A, and SHANK3 mutations, mitochondrial disorders, and FMR (fragile X) gene expansions. For families who have one child with autism, the risk of having a second child with the disorder increases by 20%. Among identical twins, if one child has autism, the sibling has a 36 to 95 percent chance of having autism.
What causes genetic mutations that cause autism?
As people age, their reproductive cells (sperm and eggs) are more susceptible to genetic mishaps. Since sperm are produced every day, they are particularly susceptible to genetic errors. A very large 2015 study funded by Autism Speaks supports the hypothesis of parental age-related mutation as the primary cause of autism. The researchers analyzed a database containing information on nearly six million children from Denmark, Israel, Norway, Sweden, and Western Australia; more than 30,000 of these children were autistic. Both increasing paternal and maternal age were associated with autism. However, the strongest causal factor associated with autism was paternal age, which makes sense, biologically speaking. Constant sperm production opens the door to frequent genetic errors, especially as men age. The correlation between increasing paternal age and autism risk has been replicated in many studies over time. In the Autism Speaks study, fathers aged 40 had a 28% increased risk of having an autistic child, and for fathers over 50, the risk increased by 66% (still a low figure, given the base rate of 1.5%). Age gaps between mother and father of more than 10 years were also associated with an increased risk of autism in this study, but the reasons behind this finding remain a mystery. Interestingly, autism rates were 18% higher among teenage mothers, possibly due to birth complications or underlying genetic factors.
Personal anecdote and observations
As an aside, when I lived in New York City during my pediatric residency, it seemed like no (or very few) Manhattan women had children before age 40. In the city, women tended to have careers first and children later. Studies have shown that increasing education levels among women correlate with an increased risk of autism in their children; the effect is dose-dependent, meaning that the more educated a woman is, the more likely her child is to have autism. In my opinion, this probably has more to do with maternal/paternal age than with access to health care providers, diagnoses, or medical services. This may explain why the autism rate is so high in New Jersey, where 1 in 41 children are currently diagnosed with autism. In fact, Alabama has the lowest autism rate in the United States.
Environmental factors and associated risks
In addition to genetic mutations, certain environmental factors increase the risk of autism. Pregnancy-related problems, such as chronic maternal illness, gestational diabetes, infections, bleeding, hypoxic-ischemic events, prematurity, and low birth weight, have all been linked to autism. The antiepileptic drug valproate is known to cause autism, as well as spina bifida and other birth defects. Severe maternal hypothyroidism is also problematic, likely because thyroid hormone influences brain development.
Some studies have shown a link between autism and prenatal exposure to organophosphate pesticides. However, there is no strong evidence to suggest that eating organically prepared foods reduces the risk of autism. Studies in North Carolina, Pennsylvania, and California have shown a link between prenatal air pollution and autism. Other toxic chemicals, such as cyanide, methylene chloride, methanol, and arsenic, have been proposed as causative agents.
Take preventive measures
On an encouraging note, large studies have recently shown that taking folic acid and/or prenatal multivitamins reduces the risk of having an autistic child; however, these results are not consistent across studies. Regular consumption of fish during pregnancy (but not fish high in mercury) has been shown to promote optimal brain development in babies and may also help prevent autism.
Since spontaneous genetic mutations can occur in people of all ages, autism is not 100% preventable; but the odds can potentially be shifted in the right direction by following the steps outlined below:
1. Avoid unwanted pregnancies by using reliable methods of birth control. Women who have unwanted pregnancies are less likely to take folic acid or prenatal vitamins before and after conception.
2. Women who wish to become pregnant must:
- Eat two to three servings of fish per week to promote healthy brain development in the womb (consumption of high-mercury fish, such as king mackerel, marlin, orange roughy, shark, swordfish, tilefish, ahi tuna and bigeye tuna should be limited).
3. Avoid pregnancies at extreme ages. If possible, women should avoid having children during adolescence or after age 40. From an autism prevention perspective, the best time to have children is between the ages of 20 and 30. It may also be helpful to discuss family planning strategies with male partners.
4. Men should try to have children before age 40. In my opinion, men who are considering having children should not keep their cell phones in their pants pocket, due to the risk of damaging sperm. Yes, I am serious. Numerous studies have shown that radiofrequency radiation, the type of low-frequency radiation emitted by cell phones, iPads, and laptops, causes stress at the cellular level and is not healthy for sperm. If you are not convinced, visit PubMed via Google and use the site to do a quick search. Type in “radiofrequency radiation and sperm” and see what comes up.
5. Consider egg and sperm banking in your 20s and 30s. People who plan to have children later in life, after a long career, should consider egg and sperm banking. For women, getting pregnant in their 40s can be difficult. Additionally, the risk of having a baby with genetic disorders increases significantly with age, especially after age 40.
6. Choose sperm donors wisely. If you are a woman and want to choose sperm from a bank, choose wisely. Do not choose sperm from male donors over 40 years old.
7. Consider the age of the partner. If the hypothesis of a parental age difference is well-founded, consider procreating with someone close to your age.
8. Consider genetic screening before conception. To rule out recessive conditions that can cause autism, individuals who are carriers of recessive genetic conditions associated with autism may be candidates for in vitro fertilization and preimplantation genetic diagnosis.
9. Discuss the risks of medications. Women should discuss the risks of basic medications with their doctor. It may be necessary to stop taking seizure medications such as valproate or replace them with other treatments during pregnancy.
10. Avoid exposure to air pollution, pesticides or other toxic chemicals. If possible, avoid these exposures before conception.
To learn more about pregnancy-related recommendations, consult the advice from Public Health France .
