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

Sex, genetics, and pregnancy complications may have an effect, but that's not all.

nurse, boy, vaccine, shot, boy getting a shot in the arm, needle

Updated on May 13, 2022.

Autism spectrum disorder (ASD) is a widely misunderstood group of neurodevelopmental disorders affecting about one in every 44 children, according to the Centers for 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 an advisory board member at Sharecare. They may not make eye contact or respond when you call their name, and often compulsively 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 necessarily direct causes, says Victoria Chen, MD, a pediatrician in New Hyde Park, New York 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.”

Known risk factors for autism

Genetics: Evidence strongly suggests there is a genetic component to autism. More than 800 genes have been associated with ASD. According to the National Fragile X Association, about 15 percent of children with autism have a genetic disorder. These can include Fragile X Syndrome, which causes intellectual disabilities, and tuberous sclerosis, which causes noncancerous tumors in the brain and other organs.

Identical twin studies—involving twins who have the same set of genes—have found that, if one sibling has autism, the other sibling is 58 to 90 percent more likely to also have ASD. A 2017 analysis of studies published in JAMA estimated the genetic association of autism to be about 83 percent.

If a family has one child with autism and then has another child, the risk of the second child also having autism is about 20 percent. When there are multiple siblings with autism, the risk of consecutive younger siblings having the disorder increases to about 36 percent, according to a 2020 JAMA article.

Genetics don’t paint the entire picture, however.

"That’s what often gets misrepresented in the media," Dr. 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 March 2021 article in Scientific Reports, children exposed to gestational diabetes were 1.4 times more likely to be diagnosed with autism.

Preterm birth, low weight at birth, Cesarean delivery, and factors 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. She notes, though, that these are 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: Parents who are older when they conceive may have a 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 fact, a 2017 study from China found that older mothers and fathers have a 41 percent to 55 percent higher risk, respectively, of having a child with autism.

Sex:­ “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 27 boys compared to 1 in 116 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. 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.”

According to the CDC, children younger than 9 months may show symptoms such as lack of eye contact, inability to express emotions like joy and surprise, and no interest in responding to their name. “That’s how early the changes are happening,” Chen says.

Raising children with ASD

If your child has autism, you may have to change your vision of what raising a child is going to look like and learn to embrace your child’s strengths and individuality.

“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 adds—and that diversity could be a big benefit for families and societies.

Critical to the development of a child with autism are diagnosis and intervention. The American Academy of Pediatrics recommends that healthcare providers should look for signs of developmental problems including autism during all baby well visits and screen for ASD at 18 months and 2 years. If there’s any indication of the condition, your child should be referred right away to a specialist, as early intervention programs can provide invaluable benefits.

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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.
UpToDate.com. Autism spectrum disorder: Terminology, epidemiology, and pathogenesis.
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.
Sandin S, Lichtenstein P, Kuja-Halkola R, Hultman C, Larsson H, Reichenberg A. The Heritability of Autism Spectrum Disorder. JAMA. 2017;318(12):1182–1184.
McDonald NM, Senturk D, Scheffler A, et al. Developmental Trajectories of Infants With Multiplex Family Risk for Autism: A Baby Siblings Research Consortium Study. JAMA Neurol. 2020;77(1):73–81.
Rowland, J., Wilson, C.A. The association between gestational diabetes and ASD and ADHD: a systematic review and meta-analysis. Sci Rep 11, 5136 (2021).
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