Sex differences in pregnancy complications and autism
This study utilizing Swedish birth registry data reveals that while male pregnancies are generally more prone to complications, the link between these complications and autism is largely independent of fetal sex, with prematurity and other factors consistently associated with autism across both sexes, ultimately identifying fetal sex as the strongest predictor of autism likelihood.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Autism is a lifelong condition that shapes how a person connects with others, communicates, and experiences the world. While scientists have long known that genetics play a major role in whether a child develops autism, the story is not written in DNA alone. The earliest stages of life, particularly the time spent growing inside the womb, also matter. During pregnancy, the placenta acts as a vital bridge between mother and baby, delivering nutrients and oxygen while filtering out waste. This organ does not function identically for every pregnancy; it responds differently depending on whether the developing baby is male or female. For decades, researchers have noticed that autism is diagnosed more often in boys than in girls, but the reasons for this gap have remained elusive. Understanding whether the physical challenges a pregnancy faces contribute to this difference could help clarify the complex mix of biology and environment that leads to an autism diagnosis.
A team of researchers in Sweden set out to investigate this question by looking at a massive collection of medical records. They examined data from over two million births, a number large enough to spot patterns that smaller studies might miss. Their work was divided into two main parts. First, they looked at the general population to see if male and female fetuses face different risks during pregnancy. Then, they focused specifically on children who had been diagnosed with autism to see if the complications their mothers experienced were different for boys and girls. By comparing siblings within the same families, the researchers could also control for factors shared by brothers and sisters, such as their mother's genetics or the home environment, to isolate the specific effects of the pregnancy itself.
The first part of the study confirmed a long-held observation: male pregnancies are generally more fragile. When the researchers compared millions of births, they found that male fetuses were more likely to be born prematurely, to require a Cesarean section, and to be born with a condition called placenta previa, where the placenta blocks the birth canal. Female fetuses, by contrast, were more likely to be born larger than average for their gestational age. This suggests that male fetuses may struggle more to adapt when the pregnancy faces stress, while female fetuses might be better at adjusting their growth to match the available resources. These differences were consistent even when the researchers looked at brothers and sisters born to the same mother, suggesting the vulnerability is tied to the sex of the fetus itself.
The researchers then turned their attention to the children diagnosed with autism. They found that pregnancy complications were indeed linked to a higher likelihood of an autism diagnosis, but the story became more nuanced when they looked at the sex of the child. In the general population, boys are much more likely to be diagnosed with autism than girls. However, when the researchers looked at children who already had autism, the usual pattern of male vulnerability disappeared. In fact, among children with autism, girls were more likely than boys to have been born prematurely. This finding suggests that the factors leading to autism in girls might be different from those in boys. For girls, the stress of a difficult pregnancy, particularly being born too early, appears to be a stronger signal for autism than it is for boys.
To understand how much of the gender gap in autism could be explained by these pregnancy issues, the team used statistical models to trace the connections. They found that the fact that male fetuses are more prone to complications like prematurity and Cesarean sections does explain a small part of why more boys are diagnosed with autism. However, this factor accounts for less than two percent of the overall difference. The vast majority of the gap remains unexplained by these specific pregnancy events. This indicates that while the physical challenges of pregnancy play a role, they are not the primary driver of the sex difference in autism rates. The study also used advanced computer modeling to predict autism likelihood based on birth records. In these models, the sex of the fetus was the single most powerful predictor, far outweighing any specific complication like high blood pressure or diabetes.
The study also distinguished between children with autism who also have an intellectual disability and those who do not. The links to pregnancy complications were generally stronger for children with both conditions. For example, being born small for their gestational age or having preeclampsia (a condition involving high blood pressure in the mother) showed a stronger association with autism when intellectual disability was also present. When the researchers compared siblings where one had autism and the other did not, some of the links seen in the general population disappeared. Complications like gestational diabetes and pregnancy-induced hypertension no longer showed a significant connection to autism when comparing siblings. This suggests that for these specific conditions, the link to autism in the broader population might be influenced by shared family factors, such as the mother's overall health or genetics, rather than the pregnancy event itself.
Ultimately, this research paints a picture of a complex biological landscape. It confirms that male fetuses are more vulnerable to certain pregnancy complications, and that this vulnerability contributes slightly to the higher number of autism diagnoses in boys. However, the study also reveals that the path to an autism diagnosis for girls may involve different mechanisms, potentially involving a heightened sensitivity to being born prematurely. The findings underscore that while the environment of the womb matters, it is only one piece of a much larger puzzle. The sex of the child remains the strongest single factor in predicting autism likelihood, pointing to deep biological differences that begin before birth and continue to shape development long after the baby is born.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.