A number of studies have previously shown that there is an association with being born during a pollen season and an increased risk of allergies. Although the pollen season is a regular annual event, there are large variations between years in pollen levels. Few studies have closely examined the significance of actual pollen content in different time periods before and after birth, but now, researchers at the Division of Occupational and Environmental Medicine at Umeå University have conducted such a study involving 110,000 pregnancies in the Stockholm area.

The researchers note that high levels of pollen exposure during the last 12 weeks of pregnancy resulted in a significantly increased risk of hospitalization for asthma symptoms in the first year of life for a child. The analysis was adjusted for factors such as maternal smoking and pollen season.

There may be several reasons for the association. High pollen exposure of pregnant with pollen allergies can have allergic reactions and asthma symptoms that may also affect the unborn child's environment and affect immune system development. It is also possible that pregnant women with severe reactions to pollen suffer complications and sometimes give birth earlier than they otherwise would have done, which in itself increases the risk of respiratory problems in the child.