Autism spectrum disorder (ASD) is a complex developmental condition characterized by challenges with social skills, repetitive behaviors, speech and nonverbal communication. The causes of autism are still not fully understood, but research suggests that genetic and environmental factors play a role. One question that has been explored by autism researchers is whether the gestational age at birth influences a child’s risk of developing autism. Some studies have found associations between late birth and increased autism risk, while other studies have not found clear links. In this article, we will explore the evidence around autistic babies being born later, look at possible reasons behind these findings, and discuss what is still unknown about how gestational age may relate to autism risk.
Key Questions
Here are some of the key questions around autistic babies being born later:
– Are autistic babies more likely to be born after 40 weeks gestation compared to non-autistic babies?
– If there is an increased risk of autism in post-term births, how much higher is the risk compared to full-term births?
– Is the association between longer gestational age and autism found consistently across different studies? Or are the findings mixed?
– Does gestational age at birth show a dose-response relationship with autism risk? i.e. is the risk higher the later a baby is born?
– Are there differences between spontaneous late births versus medically-induced late births in terms of autism outcomes?
– Are there differences between first-born versus later-born children who are post-term births in terms of autism risk?
– What are the possible biological mechanisms that could explain why late birth may increase autism risk?
Studies Investigating Links between Gestational Age and Autism
There have been a number of studies over the past 15 years investigating whether autistic children are more likely to be born post-term (after 40 weeks):
Earlier Studies Suggesting a Link
– A 2004 study by Glasson et al looked at over 128,000 births in Western Australia. It found a higher risk of autism in births after 41 weeks compared to 40 weeks.
– In 2008, a study by Durkin et al looked at over 4,700 cases of autism and found a small increase in risk for births after 42 weeks.
– A 2010 study by Kurppa et al in Finland found a 3-fold increased risk of autism in births after 42 weeks compared to 39-41 weeks.
Studies Showing Minimal or No Link
– A 2010 study by Grether et al in California did not find evidence for an association between gestational age and autism risk.
– A large 2013 Danish study by Maimburg & Vaeth found only a slightly increased risk of autism in post-term births and the authors concluded the effect was negligible.
– A 2015 meta-analysis combining multiple studies concluded there was no significant relationship between gestational age and autism.
More Recent Studies Suggesting an Increased Risk
– A 2019 Swedish study by Yang et al found a 43% higher risk of autism in births after 40 weeks compared to births at 40 weeks.
– A 2021 study by Kuzniewicz et al looked at over 100,000 births and found a 22% higher autism risk for births after 41 weeks compared to 39-41 weeks.
– A 2022 study by Gao et al analyzed over 1.2 million births in China and found increased autism risk started at gestational ages over 37 weeks and was highest after 42 weeks.
Key Findings from the Research
Based on a review of multiple studies in this area, here are some of the key findings that emerge:
– The research overall suggests there may be a small increased risk of autism in babies born post-term (after 40-41 weeks) compared to full-term births.
– However, the findings are inconsistent across studies with some studies showing minimal or no increased risk.
– The largest increases in risk are typically seen in extremely post-term births after 42 weeks gestation.
– Any increased risk of autism in post-term births is likely to be small – on the order of 20-40% relative to full-term births.
– Some researchers argue the effect is negligible and not clinically meaningful.
– First-born children and medically-induced post-term births may show slightly stronger associations with autism in some studies.
Possible Explanations for the Link
There are several possible reasons why late birth could potentially increase autism risk:
Pregnancy Complications
Post-term pregnancies have higher rates of complications like preeclampsia, placental deterioration, and need for induced labor. These complications could potentially impact fetal brain development.
Hormonal Changes
Hormone changes near the end of pregnancy could affect neurodevelopment, and may become more extreme post-term.
Genetic Factors
Shared genetic factors could predispose a child to both longer gestation and autism risk.
Environmental Stress
The intra-uterine environment may become less optimal over time, causing stress to the fetus.
Epigenetic Changes
The regulation of certain genes may be altered near the end of or after term. This may affect aspects of early brain development.
Low Birth Weight
Post-term infants are at greater risk for low birth weight which itself has been linked to increased autism risk.
Unanswered Questions
There are still important unanswered questions about the relationship between gestational age and autism risk:
– Is there a specific gestational age threshold that clearly increases risk? Findings have been inconsistent across studies.
– How important are medical induction versus spontaneous labor as factors?
– Can changes near the end of pregnancy explain the increased risk? Or are shared genetic/environmental factors the core reason?
– Does gestational age relate differently to high vs. low functioning autism? Evidence is lacking currently.
– Can the possible small increase in autism risk be mitigated through early behavioral interventions? This needs to be studied.
More large-scale prospective studies tracking gestational age, medical factors, and behavioral development over time are needed to fully understand this association.
Conclusion
In summary, there is some evidence that autistic children may be slightly more likely to be born post-term, compared to non-autistic children. However, findings remain somewhat inconsistent across studies. The research suggests gestational ages over 40-41 weeks are associated with a small (~20-40%) increased risk of autism compared to full-term births. But many studies also find no clear statistically significant relationship.
If an association exists, the underlying reasons are still unclear but could involve a combination of pregnancy complications, hormonal changes, genetic factors, and environmental stress. More research is needed to clarify the potential link between gestational age and autism, and to determine if any causal relationship exists. While interesting, based on current evidence this factor likely plays a very small role in overall autism risk.