A new study boosts suspicions that a child's risk of allergies could be linked to the season that coincides with the first three months of pregnancy.
The study, from the Institute of Health Sciences at the University of Oulu, appears in the Journal of Epidemiology and Community Health.
Researchers, headed by Kaisa Pyrhonen, cast an eye over the health records of 5920 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% 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.
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.
Dr Raymond Mullins, a specialist in immunology and allergy at the John James Medical Centre in Canberra, says while the study strengthens the link between seasonal birth and food allergy, it doesn't fully explain why it occurs.
"The relationship between seasonal birth and later manifestations of allergic disease has fascinated authors for decades, but so far, no unifying hypothesis has been presented to explain at times conflicting findings," says Mullins.
He says there are a number of factors that could boost food allergy risk, including exposure to winter infections and seasonal differences in the diet of mother or child.
"[It] may also indirectly reflect the timing of first exposure to solids four to six months later or more commonly 'allergenic' foods (e.g. peanut) 12 to18 months later."
Mullens says another possible explanation might be vitamin D deficiency.
"Fall/winter birth might also be a proxy for the effects of lesser sunlight exposure and vitamin D deficiency on the developing immune system for which there is indirect evidence at this time," he says.
"Nonetheless, [this research] is a small piece of a very large puzzle and may provide clues to be many potential contributors to food allergy risk and therefore potential targets with which to intervene in the future."