Preterm and early birth is associated with an increased risk of autism independent of genetic or environmental factors, according to new research published in Pediatrics.
Although previous studies have linked preterm births to an increased risk of autism – one 2017 study published in Cerebral Cortex found that 27.4% of the children born extremely preterm were diagnosed with autism – Casey Crump, MD, PhD, said potential causality, sex-specific differences, and association with early-term births were still unclear.
“Preterm birth had previously been linked with higher risk of autism; however, several important questions remained unanswered,” said Dr. Crump, professor and vice chair for research at the department of family medicine and community health and professor of epidemiology in the department of population health science and policy at Icahn School of Medicine at Mount Sinai New York. “To our knowledge, [our study] is the largest to date of gestational age at birth in relation to autism, and one of the first to investigate sex-specific differences, early term birth, or the influence of shared familial factors.”
Dr. Crump and colleagues examined data from more than 4 million infants born in Sweden between 1973 and 2013 who were followed-up for autism spectrum disorder identified from nationwide outpatient and inpatient diagnoses through December 2015. Children born between 22 and 27 weeks were considered extremely preterm, those born between 28 and 33 week were characterized as very to moderate preterm, and those born between 34 and 36 weeks were considered late preterm. Early-term births are characterized as infants born between 37 and 38 weeks and children born between 39 and 41 weeks were considered term births.
They found that 6.1% of those born extremely preterm were diagnosed with autism. Meanwhile, autism spectrum disorder prevalences were 2.6% for very to moderate preterm, 1.9% for late preterm, 2.1% for all preterm, and 1.6% for early term, compared with 1.4% for term birth.
The researchers’ analysis showed that preterm and early birth were associated with a significantly increased risk of autism in males and females. Children who were born extremely preterm had an approximately fourfold increased risk of autism. Researchers also found that each additional week of gestation was associated with a 5% lower prevalence of autism spectrum disorder (ASD) on average.
“The elevated risk even in [late preterm] infants is not completely surprising because a number of investigators have shown higher incidences of early cognitive, language motor and impairment, and school problems ... and psychiatric disorders, some of which may extend to adulthood,” Elisabeth McGowan, MD, who was not involved in the study, said in a solicited editorial commentary about the study.
Dr. Crump believes the association between preterm birth and autism may be because of increased inflammatory marker levels. A 2009 study published in Reproductive Sciences found that increased proinflammatory cytokine levels have been associated with the timing and initiation of preterm birth, and also have been detected in the brain and cerebrospinal fluid of individuals with autism “and may play a key role in its pathogenesis,” Dr. Crump said.
“Inflammatory-driven alteration of neuronal connections during critical periods of brain development may be central to the development of autism,” Dr. Crump explained.
However, Dr. Crump said that, although the relative risks of autism were higher in those born preterm, the absolute risk of the condition is low.
“The report by Crump is in many ways a definitive accounting of the elevated rates of ASD in preterm infants,” said Dr. McGowan, associate professor of pediatrics at the Women and Infants Hospital, Providence, R.I. “And although the impact of prematurity on brain development may be part of the causal chain resulting in ASD (or other neurodevelopmental outcomes), these factors are operating in a complex biological landscape, with pathways to ASD outcomes that can be expected to be heterogeneous.”
ASD is a developmental condition that affects about 1 in 54 children, according to the Centers for Disease Control and Prevention. Many children are not diagnosed with ASD until later in childhood, which in some cases delays treatment and early intervention. ASD may be detected as early as 18 months, but the average age of diagnosis for ASD is 4.3 years, according to the CDC.
“Children born prematurely need early evaluation and long-term follow-up to facilitate timely detection and treatment of autism, especially those born at the earliest gestational ages,” Dr. Crump said in an interview. “In patients of all ages, gestational age at birth should be routinely included in history-taking and medical records 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 autism and other neurodevelopmental conditions in those born prematurely.”
Dr. Crump and colleagues said more research is needed to understand the biologic mechanisms linking preterm birth with higher risks of autism, which “may reveal new targets for intervention at critical windows of neurodevelopment to improve the disease trajectory.”
Experts interviewed did not disclose any relevant financial relationships.