What Really Increases the Risk for Autism—and It's Not Vaccines

What Really Increases the Risk for Autism—and It's Not Vaccines

Gender and genetics can have an effect, but that's not all.

Autism spectrum disorder (ASD) is a widely misunderstood group of neurodevelopmental disorders affecting about 1 in every 59 children, according the Centers of Disease Control and Prevention (CDC). It can be diagnosed by age 2, though it commonly isn't until age 3 or 4, since parents and guardians frequently don't know the signs.

“Children with autism are often late to start talking,” says Tanya Altmann, MD, a pediatrician practicing in Southern California and Advisory Board member at Sharecare. They may not make eye contact or respond when you call their name, and often obsessively engage in repetitive behaviors. These children don’t meet age-appropriate developmental milestones, she adds.

There is no single cause of autism, and although there are factors associated with an increased risk, they are not directly causative, says Victoria Chen, MD, a pediatrician who treats children with special healthcare needs. “The simplest way to talk about risks for autism is that there are genetic factors and environmental factors and some combination of the two is causative for autism, but we don’t know enough to be able say what that combination is,” she says.

Known risk factors for autism
Genetics: Evidence strongly suggests there is a genetic component to autism. More than 600 genes have been associated with ASD. About 20 percent of children with autism have a genetic disorder, such as Fragile X Syndrome—which causes intellectual disabilities—or tuberous sclerosis, which causes noncancerous tumors in the brain and other organs.

In regard to family history, Dr. Chen says that in identical twin studies—twins that have the same set of genes—if one sibling has autism, the other sibling is 58 to 90 percent likely to also have ASD. If a family has one child with autism and they have another child, the risk of the second child also having autism is about 10 to 18 percent, she adds. When there are multiple siblings with autism, the risk of the next child having the disorder increases to about 30 percent.

However, genetics isn't the whole picture. "That’s what gets misrepresented in the media," Chen explains. "It’s a lot more complicated.” In fact, she says, a study of whole exome sequencing, a high level of genetic testing, found that two siblings with autism sometimes have different genetic mutations. “This is mindboggling,” she says.

Pregnancy-related risk factors: What happens just before, during and immediately following birth is critical. For example, taking the drugs valproic acid or thalidomide during pregnancy may raise the risk for having a child with autism, as can having gestational diabetes. In fact, according to a June 2018 study published in the Journal of the American Medical Association, having type 1, type 2 or gestational diabetes when you're pregnant is linked to a small increase—up to 2.5 percent—in the incidence of ASD. Researchers analyzed health data from more than 419,000 children and found those exposed to gestational diabetes by 26 weeks were more likely to develop autism, while those exposed after 26 weeks saw no increased risk.

Preterm birth, low weight at birth, Cesarean delivery and things related to neonatal hypoxia—when the baby doesn’t get enough oxygen at birth—all seem to increase the risk for autism in one study or another, Chen says, although they’re generally not things a mother can necessarily avoid or control.

Chen says there is one thing would-be parents CAN do: Work with an obstetrician or midwife you trust. When that person wants to escalate the level of care to avoid neonatal distress or neonatal hypoxia, she adds, you want to have someone you trust to make these critical decisions. “When physicians choose doctors," she explains, "they ask themselves, ‘If I was in an emergency, would I want this person making those decisions with me?’”

Age of parents at conception: Mothers and fathers who are older when they conceive may have a somewhat higher risk of having a child with autism, Chen says—particularly older fathers. The odds rise after men turn 30 and increase with age. In one 2010 study in Molecular Psychiatry, researchers found men age 50-plus were 2.2 times more likely to have a child with ASD than men under 30.

Gender:­ “There’s good data that there’s a higher prevalence of autism in males, although it’s not completely understood why that is,” Chen says. “Autism tends to occur in about 1 in 38 boys compared to 1 in 151 girls.”

What's not a risk factor
Ever since a small, discredited 1998 study suggested a link between autism and childhood vaccines, there have been questions about the connection. Dr. Chen says there are very good epidemiological studies that have examined it extensively. “There’s nothing there,” she explains. No link is known to exist between the two.

Furthermore, she adds, “The changes that occur in the brain that are associated with autism happen really, really early, in utero and when children are born, long before they receive vaccines.” In fact, recent studies show that even infants may exhibit signs of autism, such as lack of eye contact, that we didn’t necessarily pick up in the past. “That’s how early the changes are happening,” she says.

Raising children with ASD
If your child has autism, you may have to change your vision of what raising a child was going to look like, and base it on the reality of the child you actually have. “Some of the blessing of having a child with special healthcare needs is that you really hone into what’s important in your life and what’s not," says Chen. "You have the freedom to choose what you value.” We underappreciate some of the neurodiversity in milder forms of autism, Chen continues—and that diversity could really be a big benefit for families and societies.

Critical to the development of a child with autism is diagnosis and intervention. The American Academy of Pediatrics recommends healthcare providers look for signs of developmental problems including autism during all baby well visits, and screen for ASD at 18 and 24 months. If there’s any indication of the disorder, treatment should be referred right away, as early intervention programs can provide invaluable benefits.  

Medically reviewed in July 2018. Updated in December 2019.

National Institute of Neurological Disorders and Stroke. “Autism Spectrum Disorder Fact Sheet.”
Al-Mubarak B, Abouelhoda M, et al. “Whole exome sequencing reveals inherited and de novo variants in autism spectrum disorder: a trio study from Saudi families.” Sci Rep 7, 5679 (2017).
Centers for Disease Control and Prevention. “Research on Autism Spectrum Disorder,” “Key Findings: Population Attributable Fractions for Three Perinatal Risk Factors for Autism Spectrum Disorders, 2002 and 2008 Autism and Developmental Disabilities Monitoring Network,” “Vaccines Do Not Cause Autism,” “MMWR: Prevalence of Autism Spectrum Disorders --- Autism and Developmental Disabilities Monitoring Network, Six Sites, United States, 2000,” “Data & Statistics on Autism Spectrum Disorder,” “Prevalence of Autism Spectrum Disorder Among Children Aged 8 Years — Autism and Developmental Disabilities Monitoring Network, 11 Sites, United States, 2014.” “Autism spectrum disorder: Terminology, epidemiology, and pathogenesis”
Rochman, Bonnie. “For Siblings of Autistic Kids, Risk Is Far Higher Than Thought.” TIME online. August 15, 2011.
Loomes R, Hull L, Mandy WPL. “What Is the Male-to-Female Ratio in Autism Spectrum Disorder? A Systematic Review and Meta-Analysis.” J Am Acad Child Adolesc Psychiatry. 2017 Jun;56(6):466-474. 
Hultman C, Sandin S, et al. Advancing paternal age and risk of autism: new evidence from a population-based study and a meta-analysis of epidemiological studies. Mol Psychiatry 16, 1203–1212 (2011)
Lipkin PH,  Macias MM, et al. “Promoting Optimal Development: Identifying Infants and Young Children With Developmental Disorders Through Developmental Surveillance and Screening.” Pediatrics. December 2019, e20193449.
Steenhuysen, Julie. “New autism guidelines focus on early diagnosis, treatment.” Reuters. December 16, 2019

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