Stockholm -- Babies are more likely to suffer from asthma if their mothers were exposed to pollen during the last three months of pregnancy, Swedish scientists said on Monday.

Scientists have already linked asthma, which has a strong allergic component, to changes in the immune system. A study at the University of Umea showed that environmental factors such as pollen could be equally important for babies even before birth.

Epidemiologist Bertil Forsberg and his colleagues studied 111,702 babies conceived between 1988 and 1995 and born in the Stockholm region. The study monitored them for the first year after birth, during which 923 were hospitalized for asthma.

"From a comparison of babies who early in life become asthmatic and those who do not, it is clear that maternal pollen exposure in the last 12 weeks of pregnancy plays a major role," Forsberg told Reuters.

Pollen levels vary during different months of the year. The researchers did not specifically look at the impact of the month of birth on asthma risk, but said it could also be a factor.

"The exposure level seems to be more important than the time of year (the baby was born)," Forsberg said.

This is partly because pollen levels vary from year to year as well, and exposure can be extremely heavy in some years but modest in others.

After the initial study, researchers monitored the children up to their second birthday and came up with similar results, he said.

The team plans to study children up to the ages of four or six years to see if the pollen exposure was still a risk factor.

Forsberg said childhood asthma could be linked to other factors such as the length of pregnancy and exposure to other allergens both during pregnancy and after birth.

The study is due to be presented to the 12th annual congress of the European Respiratory Society in Stockholm on Monday.