Researchers cast an eye over the health records of 5,920 children born between 2001 and 2006 in the southern Finnish province of South Karelia, 961 of whom were given skin tests for allergies by the age of four.
Among these, 10 per cent of those who were born in October and November tested positive, twice the proportion of those born in June and July. Allergy sensitivity among the October-November babies was especially strong for milk and eggs.
The possible reason, suggest the investigators, is that the October-November babies were exposed to high concentrations of birch and alder pollen while they were at a key stage of foetal development — in the 11th week of pregnancy.
The peak pollen period for these trees is in April and May.
Conversely, the lowest pollen period was December and January, which explains why children born in June and July had the lowest allergy sensitivity.
Exposure in the womb to powerful allergens at a crucial early phase of pregnancy could affect the development of the baby’s immune system, although how this happens is unclear, the researchers say.
The study was headed by Kaisa Pyrhonen of the Institute of Health Sciences at the University of Oulu, was published on Wednesday in a British publication, the Journal of Epidemiology and Community Health.
It boosts previous findings in studies carried out in Sweden, Japan and the Netherlands that children born in the northern hemisphere’s autumn or winter are more prone to eczema and asthma-like wheeze, and have higher levels of antigens in the blood than children born in spring or summer.