First-born babies may be programmed in the womb to have a higher risk of asthma and allergy, research suggests.
A University of South Carolina led team carried out tests on more than 1,200 newborns from the Isle of Wight.
They found first borns were more likely to carry a gene variant which raised their risk of allergy.
The study, to be presented to the American Thoracic Society, found this seemed to translate to a higher risk of allergy through to the age of ten.
The researchers measured levels of an antibody called Immunoglobulin E (IgE) in the babies' umbilical cord blood.
This is known to play a key role in the development of allergic responses.
First born babies were more likely to have high levels of IgE, and those that did were also more likely to show signs of an allergic response when they were subsequently tested, using a skin prick test, at the age of four and ten.
The researchers also believe they may have pinned down the genetics behind the difference to variations in a gene called IL13, which controls production of a stress hormone called a cytokine, which in turn influences levels of IgE.
They found that first born children were more likely to carry a variant of IL13 which raises levels of the cytokine, and in turn IgE.
The researchers believe that birth order may affect expression of the gene during the foetal differentiation and development.
Lead researcher Dr Wilfried Karmaus said: "We were not surprised that birth order had an effect on the development of the immune system, but were surprised that this interaction persisted at least through age ten.
"Our findings add to the evidence that allergic reactions are programmed during pregnancy and then effect the disease in later life.
"This finding may partially account for the increasing prevalence of asthma and allergies in children in the last 30 years, primarily seen in the western world, as developed nations' birthrates continue to decline."
Dr Karmaus suggested that a first born experienced different conditions in the uterus from subsequent siblings.
He said that if a way to modify those conditions could be found to make them more like those experienced by later born children, then potentially it might be possible to prevent 20-30% of all cases of asthma and allergy.
Previous research has suggested that high levels of IgE in umbilical cord blood are linked to raised levels of a chemical called organochlorine in the placenta, which disrupts hormone production.
It is thought that organochlorine levels dip in subsequent pregnancies.
Mike Thomas, chief medical adviser to the charity Asthma UK said: "It has been known for some time that the risk of developing asthma and allergies is higher in first born children, but why this should be so hasn't been clear.
"It has been thought that increased exposure to infections passed on by older siblings after birth may reduce the risk; this new study however suggests that before-birth influences on first-born children while still in the womb may affect the way the immune system develops and may at least partly explain the increased risk.
"This may allow improved understanding of the complicated early-life factors underlying asthma and so may potentially open up ways of preventing asthma, although this still lies well in the future."
umbilical cord blood 臍帶血
foetal = fetal 胎兒的,胎的
account for 說明,對…負責任