A study looking at four decades of data on millions of children is showing just how much the timing of a child’s birth influences their risk of developing autism.

The research published online this week in the journal Pediatrics finds that the prevalence of autism is four times higher in kids born extremely preterm. The risk level remains elevated in all children born early, but declines the closer a baby is to full term at birth.

Increased odds of autism persisted for children born early whether they were male or female and regardless of genetic or environmental risk factors, researchers with the Icahn School of Medicine at Mount Sinai said.

The study included data on more than 4 million people born in Sweden between 1973 and 2013 from the Swedish Medical Birth Register. Researchers looked at records through 2015 to determine if the individuals were diagnosed with autism and then cross-referenced that information with each person’s gestational age at birth. They also analyzed the data to identify siblings in order to determine if genetics played a role.

Autism prevalence was 6.1% for children born extremely preterm — between 22 and 27 weeks gestation — according to the findings. Meanwhile, the developmental disorder affected 2.6% of those born at 28 to 33 weeks, 1.9% of children born at 34 to 36 weeks, 1.6% who arrived at 37 to 38 weeks and 1.4% of babies considered full term who were born between 39 and 41 weeks.

Researchers said the findings suggest that “preterm and also early term birth should be recognized as independent risk factors for ASD.” And, they said that there should be early evaluation and long-term follow-up for children born premature — especially those born at the earliest stages — to ensure that kids with autism are identified and able to access treatment.

“Our findings provide further evidence that gestational age at birth should be routinely included in history taking and medical records for patients of all ages to help identify in clinical practice those born preterm or early term. Such information can provide additional valuable context for understanding patients’ health and may facilitate earlier evaluation for ASD and other neurodevelopmental conditions in those born prematurely,” the researchers wrote in their findings.