Pregnant women with asthma are more likely to experience symptoms of the condition if they are carrying a baby girl rather than a baby boy, say Australian researchers.

The research team, led by Dr Peter Gibson from John Hunter Hospital in Newcastle, New South Wales, studied a group of 151 pregnant women, which included 33 who did not have asthma, 38 who had asthma but did not use inhaled steroids during pregnancy, and 80 who had asthma and used inhaled steroids every day during their pregnancies.

They found that 60% of the women with asthma who were carrying a male foetus remained symptom-free throughout their pregnancies and did not report any night-time breathing problems.

However, although 61% of asthmatic women carrying a female foetus were symptom-free at 18 weeks into their pregnancies, by 30 weeks only 28% remained symptom-free.

The use of inhaled steroids increased significantly among asthmatic women carrying a female foetus, but did not change in those pregnant with a male.

While the reason for this observed difference remains unknown, Dr Gibson told delegates at the International Conference of the American Thoracic Society, held in Orlando, Florida, that a female foetus may produce a substance in response to the inflammation of the asthmatic mother’s airways that somehow worsens the condition.

It is very important for pregnant women to treat their asthma, said Dr Gibson. “Asthma is dangerous to the mother and baby,” he explained. “If a mother has a severe asthma attack, it can result in reduced oxygen to the baby, which is needed for normal growth and development.”

However, he said: “The good news is that the vast majority of pregnant women with asthma can control their asthma with treatment.” He added that inhaled steroids are generally considered safer for the foetus than oral steroids.