High pollen exposure in the last three months of pregnancy significantly increases a baby's risk of serious asthma attacks, research has shown.
A Swedish study of more than 110,000 pregnancies found that high pollen count was associated with a 35pc increased risk of infants being taken to hospital because of asthma.
Exposure to high pollen levels during the first three months of life appeared to reduce the risk, but only among children of heavy smoking mothers.
Scientists checked pollen levels in central Stockholm and matched the results with medical records.
For each child, pollen count was assessed during the first and last 12 weeks of pregnancy, as well as the first 12 weeks of life.
Researchers took account of a range of factors that could have swayed the results, including mothers' smoking habits, infant gender, stage of pregnancy at birth, and season of birth.
The findings are reported today in the journal Allergy, Asthma and Clinical Immunology.
A number of possible reasons for the association are given.
Women with allergies may have reactions to pollen that affect the unborn baby's environment and affect immune system development, say the researchers.
It is also possible that pregnant women with severe reactions to pollen suffer complications that affect the child, it is claimed.
The scientists, led by Dr Adrian Lowe, from the University of Umea, wrote: "It is likely that pollen sensitised mothers exposed to high levels of pollen during pregnancy are at increased risk of symptoms and asthma exacerbations. This may in turn change the intrauterine environment..
"Alternatively, symptomatic mothers during high pollen periods may have an increased risk of pregnancy complications, including pre-eclampsia, pre-term birth and impaired foetal growth, which could influence the risk of wheezing illness.
"Further work is required to elucidate exactly how pollen exposure may prime the foetal immune system towards severe respiratory illness in early life."