Asthmatic women pregnant with a female baby suffer worse symptoms than those carrying a boy, research suggests.
Asthma attacks can reduce oxygen supply to the baby
The Australian researchers suggest the female foetus could produce a substance that could worsen their mother's asthma.
The research was presented to the American Thoracic Society conference in Orlando, Florida.
The conference also heard that pregnant women who eat oily fish may protect their babies from developing asthma.
Scientists from the John Hunter Hospital in Newcastle, Australia followed 118 pregnant women with asthma.
Steroids control the inflammation of the airways that occurs in asthma.
It was found that almost two thirds of asthmatic women who were pregnant with a male foetus were symptom-free throughout their pregnancy.
However, while 61% of asthmatic women pregnant with a female foetus were symptom-free at 18 weeks, by 30 weeks only 28% had no symptoms.
Night-time symptoms increased significantly from 18 to 30 weeks in women pregnant with a female foetus. Those carrying boys had no night-time breathing problems.
Women with female foetuses also used inhaled steroids more as the pregnancy progressed. Use of medication did not change in those pregnant with boys.
Researchers say that the reason for the difference is unknown.
But they say it could be that the female foetus may produce a substance in response to the inflammation of the asthmatic mother's airways that somehow worsens the asthma.
A spokeswoman for Asthma UK told BBC News Online: "We know that in about one third of pregnancies, asthma symptoms can worsen.
"We don't yet understand the mechanisms involved but this study is one of many that have suggested an association between asthma and hormone levels.
"What is important is that pregnant women with asthma continue to control their symptoms effectively, and if symptoms do get worse they visit their GP."
In a separate study, US researchers told the conference pregnant women who eat oily fish, such as salmon and trout, may help protect their children against developing asthma.
The team from University of Southern California, Los Angeles looked at five-year-olds studied as part of the Children's Health Study and interviewed their mothers about their pregnancy diet.
They found that the children of asthmatic women who ate fish such as salmon and trout during their pregnancy were, on average, 71% were less likely to develop asthma themselves than those whose mothers did not.
The more fish the women ate, the less likely her child was to develop asthma.
The finding follows earlier studies which showed eating oily fish could reduce a person's asthmatic symptoms.
But the California team found children whose mother ate fish fingers during pregnancy may be at increased risk of developing asthma.
They suggest this could be because fish fingers are deep-fried and contain omega-6 fatty acids, which encourage inflammation of the airways,
Professor Frank Gilliland, who led the research, said: "Oily fish contain omega-3 fatty acids, which appear to be anti-inflammatory, and lead to the reduced potential for developing asthma and allergies."
He added; "A family history of asthma is a very strong risk factor for a child developing asthma.
"It appears that oily fish interacts with the genes involved in the predisposition to develop asthma, and somehow reduces the risk."
Asthma UK said there was increasing evidence that how a child's immune system developed in the womb was a key factor influencing whether or not they went on to develop asthma.
The spokeswoman said: "The mother's diet and the nutrients passed to the growing baby are likely to play an important role in this process .
"This study begins to tease out some of the dietary factors that may be important."
Dr John Harvey of the British Thoracic Society said: "We have known for some time that eating oily fish can bring health benefits such as protection from heart disease, arthritis, psoriasis and dementia as well as helping to protect adults from asthma."