Babies born prematurely, even just a couple weeks early, may be at increased risk of autism, a large new study suggests.

It's long been known that autism, a developmental brain disorder, is more common among children who were born preterm -- before the 37th week of pregnancy.

Researchers said the new study, of more than 4 million people, gives a clearer breakdown of the risks associated with different degrees of prematurity.

It also points to a slightly higher prevalence of autism among children who were born just a couple weeks before their due date -- what doctors call "early term."

"Most children born preterm do well," stressed lead researcher Dr. Casey Crump, a professor at Mount Sinai's Icahn School of Medicine in New York City.

But this study, he said, strengthens the evidence that early birth is a risk factor for autism.

His team found that among people born full term -- between the 39th and 41st weeks of pregnancy -- 1.4% had been diagnosed with autism.

Among people born earlier, there was a range of risk. Autism prevalence was highest, at 6%, among those born extremely preterm (between weeks 22 and 27 of pregnancy), but was also elevated among those born more moderately preterm or early term.

Crump said babies and children who were born early should have their development tracked, to catch any delays as soon as possible.

Autism is a developmental brain disorder that affects about 1 in 54 children in the United States, according to the U.S. Centers for Disease Control and Prevention. The disorder is complex, and varies widely from person to person.

Some children have milder problems with socializing and communicating, while others are profoundly affected -- speaking little, if at all, and having repetitive behaviors. Some children have intellectual disabilities, while others have above-average IQs.

The causes of autism are complex and not fully understood, said Dr. Zsakeba Henderson, deputy chief medical and health officer for the nonprofit March of Dimes.

Like Crump, she stressed that most preemies do not develop autism.

But, Henderson said, "these findings do underscore the importance of doing everything we can to prevent babies from being born too early."

That, she added, includes avoiding labor induction before the 39th week unless there is a medical reason.

It's not completely clear why early birth can raise autism risk, Crump said. But studies show that preterm infants can harbor markers of body-wide inflammation -- a characteristic that may persist into childhood.

Inflammation affecting the brain, Crump said, could be one way preterm birth contributes to autism.

The findings -- published Aug. 11 in the journal Pediatrics -- are based on health records from more than 4 million people born in Sweden between 1973 and 2013.

Of all those born extremely preterm, 6.1% were diagnosed with autism at some point. That compared with 2.6% among people born more moderately preterm (between weeks 28 and 33 of pregnancy), and 1.9% among those born "late" preterm (between weeks 34 and 36).

Among people born in the 37th or 38th week of pregnancy (early term) autism prevalence was 1.6%.

The huge database allowed the researchers to compare siblings -- which, Crump said, helps account for genetic and family factors that might affect autism risk.

Even then, the researchers found, preterm and early-term birth were tied to a higher autism risk.

"That suggests a causal relationship," Crump said.

Dr. Elisabeth McGowan, of Women & Infants Hospital of Rhode Island in Providence, wrote an editorial published with the study. "This study gives us a good, definitive accounting of the prevalence of autism across the spectrum of preterm and early-term birth," she said.

McGowan said it's important for parents and pediatricians to be aware early birth is a risk factor for autism.

That might prompt earlier screening when a child shows problems with language or social development, she said.

McGowan agreed the findings should not "alarm" parents. But, she said, if they have any concerns about their child's development, they should not hesitate to talk to their pediatrician.

Henderson said that preventing as many preterm births as possible is key, since those babies have increased risks of other developmental disabilities, health conditions such as asthma, and problems with hearing or vision.

Among the solutions, Henderson said, is improving women's access to prenatal and post-childbirth care, through measures like extending Medicaid coverage.