Autism develops from a combination of genetic predisposition and environmental influences that shape the brain during pregnancy and early life. No single factor “gives” a child autism. Instead, dozens of genetic and prenatal variables interact in ways researchers are still mapping out. Current CDC data shows about 1 in 31 eight-year-olds in the United States has been identified with autism spectrum disorder.
Genetics Play the Largest Role
Genetic factors account for an estimated 40 to 80 percent of autism risk. That wide range reflects how complex the genetics are: hundreds of genes contribute small amounts of risk, and the combination differs from one person to the next. In roughly 2 to 4 percent of cases, a single rare gene mutation or chromosomal change is responsible, often as part of a broader syndrome that affects other parts of the body too.
Twin studies make the genetic contribution especially clear. When one identical twin has autism, the other twin also has it roughly 70 to 90 percent of the time. For fraternal twins, who share about half their DNA like any siblings, the rate drops dramatically, likely close to the general sibling recurrence risk of about 3 percent. That enormous gap between identical and fraternal twins is one of the strongest pieces of evidence that genes drive most of the risk.
Having a family history of autism increases the chances, but most children with autism are born to parents who don’t have a diagnosis themselves. Many of the relevant genes aren’t “autism genes” in a simple sense. They’re genes involved in how brain cells communicate and connect during development. When enough of these variants combine in one person, they can shift brain development in ways that produce autistic traits.
Brain Development Before Birth
The biological roots of autism begin forming during pregnancy, not after a child is born. The brain starts taking shape about four weeks after conception, and measurable differences in brain growth have been detected by ultrasound as early as 20 weeks of gestation. Research has found that children later diagnosed with autism sometimes show overgrowth in head circumference during the second and third trimesters.
One key process involves synaptic pruning, the brain’s way of trimming excess connections between neurons to build efficient circuits. In typical development, specialized immune cells in the brain (called microglia) remove unneeded synapses during late pregnancy and early childhood. In autism, this pruning process appears to work differently. Animal studies show that when microglia don’t function properly, the brain retains a higher density of connections, which can change how signals flow between brain regions. This doesn’t mean the autistic brain is “broken.” It means the wiring develops along a different trajectory, one that leads to the pattern of strengths and challenges characteristic of autism.
Prenatal Environment and Maternal Health
While genes set the foundation, conditions during pregnancy can influence whether genetic risk translates into autism. Several prenatal factors have been linked to modestly increased odds.
Infections during pregnancy are among the most studied. A large Danish study found that viral infections severe enough to require hospitalization during the first trimester were associated with a 2.8-fold increase in autism risk. Bacterial infections in the second trimester carried a 1.4-fold increase. Fever during pregnancy, independent of the type of infection, was linked to a 2.1-fold increase in odds, though mothers who took fever-reducing medication showed a weaker association.
The mechanism connecting infection to autism involves the mother’s immune response. When the body fights an infection, it produces inflammatory signaling molecules. These molecules can cross the placenta and reach the developing fetal brain, where they activate the brain’s own immune cells and interfere with normal processes like neuron migration and synapse formation. In animal studies, blocking specific inflammatory signals prevented the social behavior changes typically seen in offspring exposed to maternal immune activation.
Other prenatal factors associated with increased risk include maternal obesity, gestational diabetes, and exposure to certain environmental toxicants. These are statistical associations, not guarantees. Most mothers who experience these conditions have children without autism.
Parental Age
Both maternal and paternal age at conception affect autism risk, with older parents carrying somewhat higher odds. Compared to mothers aged 25 to 29, mothers over 40 had 51 percent higher odds of having a child with autism. The effect of paternal age depends partly on the mother’s age. Among mothers under 30, fathers over 40 showed a 60 percent elevated risk compared to fathers aged 25 to 29. The highest-risk combination in one large study was fathers under 25 paired with mothers aged 35 to 39, which carried roughly double the odds compared to the reference group.
The reason likely comes down to the accumulation of new genetic mutations in sperm and eggs over time. Sperm cells in particular acquire more spontaneous mutations as men age, because sperm-producing cells divide continuously throughout life. These new mutations can affect genes involved in brain development.
Birth Complications and Prematurity
Being born too early increases autism risk, with the degree of prematurity making a significant difference. Babies born extremely preterm (22 to 27 weeks) have an autism prevalence of about 6.1 percent, nearly four times the rate of full-term babies at 1.4 percent. For all preterm births combined (before 37 weeks), the risk is roughly 40 percent higher than for full-term births. Babies who are small for their gestational age carry about a 29 percent increased risk as well, even after accounting for prematurity.
Premature birth exposes the brain to the outside environment during a critical window when it’s still forming connections and pruning synapses. The stress and medical complications that often accompany prematurity may also play a role.
Epigenetics: Where Genes Meet Environment
Epigenetics helps explain how environmental exposures can influence gene activity without changing the DNA itself. Chemical tags attach to DNA and essentially turn genes up or down. These tags are sensitive to conditions during pregnancy, including infection, nutrition, and toxic exposures.
For example, researchers have found specific epigenetic changes in the blood of babies whose mothers had chronic asthma during pregnancy, and the altered genes overlap with genes found to be disrupted in autistic brain tissue. In animal studies, exposure to certain chemicals during pregnancy produced autism-like behavior in offspring, and those behavioral changes persisted for at least three generations through epigenetic inheritance. Folic acid intake may also matter: a genetic variation that affects how the body processes folate has been linked to higher autism rates in countries that don’t fortify foods with folic acid.
Epigenetics is essentially the bridge between “nature” and “nurture.” It’s one reason why identical twins don’t always share an autism diagnosis despite having the same DNA. Subtle differences in their prenatal environment can lead to different epigenetic patterns and different outcomes.
Vaccines Do Not Cause Autism
A 2025 analysis from the World Health Organization reviewed 31 studies published between 2010 and 2025, drawing on data from multiple countries, and confirmed no causal link between vaccines and autism. This is consistent with conclusions the WHO has reaffirmed repeatedly since 2002. Neither the vaccines themselves, nor the trace amounts of aluminum or thimerosal used as preservatives, increase autism risk. The original 1998 study that claimed a connection was retracted due to fraudulent data, and its author lost his medical license.
The timing of autism’s first visible signs, typically around 12 to 24 months, happens to coincide with the childhood vaccination schedule. That coincidence fueled the myth, but decades of large-scale research involving millions of children have consistently found no connection.
Why There’s No Single Cause
Autism emerges from a unique combination of factors in each person. One child’s autism may be driven primarily by a strong genetic load inherited from both parents. Another’s may involve fewer genetic risk factors but significant prenatal exposures that tipped the balance. In a small percentage of cases, a single identifiable genetic mutation explains the diagnosis. For most, it’s the cumulative effect of many small influences acting on a developing brain during a narrow window of vulnerability. No parent’s behavior or choices during pregnancy “cause” autism in any straightforward sense. The interplay of dozens of genetic and environmental variables makes it effectively impossible to point to one moment or one decision as the reason a child develops autism.