Date Revised: August 23, 2021
People who were born prematurely are much more likely to be diagnosed with autism than people who were born on time, according to a huge new study.
The earlier a baby is born, the higher the likelihood of having autism, according to the study in Pediatrics.1 Autism was later diagnosed in the following percentage of babies who were born at these stages of pregnancy1:
- 6 percent of babies born between 22 and 27 weeks
- 2.6 percent of babies born between 28 and 33 weeks
- 2 percent of babies born between 34 and 36 weeks
- 1.4 percent of babies born around 40 weeks, which is on time
For the study, researchers reviewed the records of more than 4 million people born in Sweden from 1973 to 2013. The study is believed to be the largest of its kind so far. It adds to the evidence from previous studies suggesting that autism is more common in premature babies, or preemies.
The autism rate increased for each additional week that a baby was born early, and that was true even for children who were delivered only a few weeks before their due date. Children who were born at 37 to 38 weeks, which is not considered premature, had a slightly higher rate of autism than those born closer to 40 weeks.
Doctors know that premature, or preterm, birth increases the risk of medical complications and death. The earlier a baby is born, especially before 32 weeks, the greater the chance of breathing, developmental, vision, or hearing problems, according to the U.S. Centers for Disease Control and Prevention (CDC).
Some smaller studies of prematurity found a higher autism rate than the Swedish study did.2, 3 In one analysis, researchers in Australia found that 7 percent of premature babies were later diagnosed with autism.2 The researchers analyzed the results of 18 scientific studies that included 3,366 premature and low-birthweight babies in various countries.
The link between prematurity and autism does not mean that one necessarily causes the other.
“The two may be seen together but caused by another factor, such as a gene abnormality,” says Paul H. Lipkin, M.D., a pediatric neurodevelopmental specialist and director of medical outpatient services at Kennedy Krieger Institute in Maryland. “Is a gene abnormality causing preterm birth; is it causing the neurodevelopmental problems seen in autism?” asks Lipkin, who was not involved in that study.
Genes and other factors may contribute to autism, together or separately. The stress of prematurity on a developing baby’s brain may work together with a “biological vulnerability” to increase the likelihood of autism in some children, according to the Australian researchers’ study.2
In the study from Sweden, researchers considered family and other factors that could increase the likelihood that a child will have autism. Their analysis suggests that premature birth itself may contribute to autism in boys and girls.1
In 2019, 1 in 10 babies were born prematurely in the United States, according to the CDC. Being pregnant with twins or multiple babies, having a history of premature delivery, pregnancy complications, smoking, and substance use each increase the risk of an early delivery. But some mothers give birth prematurely even when none of these factors apply to them. Researchers are interested in possible genetic and environmental factors that affect preterm birth.4
About 12 percent of SPARK research participants who have autism were preemies, according to information they or their parents shared. Almost one-fifth of that group was very premature, having been born at 31 weeks or earlier.
Other pregnancy-related factors may contribute to the chance of a child having autism. These factors include having older parents, prenatal exposure to some pesticides or air pollution, very low birth weight, birth complications, and a mother’s diabetes, immune system problems, or obesity during pregnancy, according to the National Institute of Environmental Health Sciences (NIEHS). “But these factors alone are unlikely to cause autism. Rather, they appear to increase a child’s risk for developing autism when combined with genetic factors,” NIEHS says.
Read “The Preemie with a Fighting Spirit” about a SPARK participant who was born at 27 weeks.