First-born babies are more likely to develop asthma and allergies than those with older siblings because of genetic changes that occur in pregnancy, US research suggests.
First-borns were found to have a variation in the IL-13 gene, which doubled the production of serum immunoglobulin E (IgE) levels, a known indicator of allergic development.
For the 10-year study, the researchers recruited 1,212 newborns and ascertained their birth order.
Samples of cord blood were collected from each child to determine levels of IgE.
Skin prick tests were also taken at the ages of four and 10 to determine sensitivity against a range of allergens.
First-borns had 1.88 times more IgE in their blood than other children and were also more likely to have a positive skin prick test at the ages of four and 10.
Further, genotype testing on 925 of the children revealed that first-born babies had a variation in a single nucleotide of the IL-13 gene, which changed an amino-acid and caused an increased production of IgE.
Lead researcher Dr Wilfried Karmaus, from the department of epidemiology at the University of South Carolina, said the findings showed that allergic reactions are programmed during pregnancy.
'We believe that birth order affects gene expression during fetal development,' he said. 'This prenatal programming may persist at least into childhood, if not adulthood.
'These findings could partially account for the increasing prevalence of asthma and allergies in children in the past 30 years, primarily seen in the western world, as birth rates continue to decline.
'It might also indicate that there are ways to reduce the chances of first-borns developing asthma and allergy by changing the conditions in the uterus,' Dr Karmaus added.
The findings were presented earlier this week at the annual American Thoracic Society Conference being held in Toronto, Canada.
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