Shortness of breath. Persistent coughing. Repeated chest infections. For many families, these are not passing symptoms but the early signs of childhood asthma, one of the most common long-term illnesses affecting children worldwide. Now, new research suggests that the roots of this condition may lie much earlier than previously thought—deep within the biology of pregnancy itself.

In a striking development, researchers from the University of Copenhagen and the Copenhagen Prospective Studies on Asthma in Childhood (COPSAC) have identified a specific fatty acid molecule in pregnant women’s blood that appears to influence whether their child develops asthma. The molecule, known as 12-HETE, may act as a quiet but crucial signal shaping a baby’s immune system even before birth.

“For the first time, we show that a specific fatty acid molecule in pregnant women is linked to the child’s risk of developing asthmatic bronchitis: If the mothers do not have a measurable amount of this molecule in their blood during pregnancy, there is a higher occurrence of childhood asthma in the child’s first year of life,” says asthma researcher and clinical professor Bo Chawes.

The team followed several hundred mothers and their children for a decade, analysing blood samples, tracking respiratory health, and even examining the bacterial communities in the children’s airways. What they found was difficult to ignore. Children born to mothers without detectable levels of 12-HETE had a 62 per cent higher risk of developing asthma during their first ten years of life compared to those whose mothers had measurable levels.

But the story does not stop there. These children also showed a pattern of more frequent respiratory infections early in life. In simple terms, their lungs seemed more vulnerable. Scientists believe this may be linked to how the immune system develops in the womb.

“As early as when the child is one month old, we can see an entirely different bacterial composition and immune response in their airways if we cannot measure 12-HETE in the mother's blood. And we know that these factors are crucial for the risk of developing asthma,” says Bo Chawes.

He explains that 12-HETE may help guide the early training of the immune system. Without it, the system may not respond effectively, leaving the child more prone to infections and inflammation in the airways. This condition, in turn, may set the stage for asthma.

The findings also add a new layer to the ongoing debate around omega-3 supplements during pregnancy. These fatty acids, commonly found in fish oil, have long been studied for their potential to protect children from asthma. However, the new research suggests that the benefit may not be universal.

“We show that 12-HETE is crucial for whether omega-3 supplementation during pregnancy has a protective effect against childhood asthma. In other words, it is not certain that all pregnant women will benefit from the same supplements, but in the future, we may be able to identify those who will gain the greatest benefit,” says Bo Chawes.

In their controlled trial, omega-3 supplements reduced early childhood asthma by 58 per cent—but only in children whose mothers had measurable levels of 12-HETE. For others, the supplements made little difference. This nuanced finding challenges the notion of universal advice during pregnancy.

Still, researchers urge caution. The study shows a strong association, not direct cause and effect. More work is needed to define exact thresholds and translate these findings into clinical practice.

"Childhood asthma is very common, so it's vital to find out what causes it and how to prevent it." With our study, we have shown that 12-HETE could potentially function as a biomarker and thus represent the first step toward a more personal and precise prevention of childhood asthma,” says Bo Chawes.

Currently, the discovery presents a promising avenue. A simple blood marker, if validated, could one day help doctors tailor advice during pregnancy—moving from broad recommendations to precise, personalised care. This shift could make a significant difference for millions of at-risk children.