The chances of a child developing asthma or other allergies may largely be fixed by the time they are born, a study suggests.
Newborns already have a functioning immune system
Researchers found babies with high levels of antibodies in their umbilical cord blood were more at risk.
Antibodies are a sign that the immune system has begun to respond to the irritants that cause allergies.
The research, by the David Hide Asthma and Allergy Centre in Isle of Wight, is published in Thorax.
The researchers analysed data on cord blood serum samples taken from more than 1,300 children born between 1989 and 1990.
Each sample was measured for levels of a chemical called IgE, which is produced by the immune system in response to the presence of allergens, such as pet waste, house dust mites and grass pollen.
The children were also assessed at the ages of one, two, four and 10 years of age to find out if they had developed allergies and/or asthma.
By the age of four, one in five children had become sensitised to allergens, and by the age of 10 more than one in four (27%) had done so.
Children who had high umbilical cord blood levels of IgE at birth were around twice as likely to have become sensitised.
One in 10 of the children had been diagnosed with asthma by the time they were one to two years old, and by the age of four, one in seven (15.2%) of the children had asthma. Almost 13% of the children had asthma by the time they were 10.
High IgE cord blood levels were not associated with the development of asthma up to the age of four.
But children with high IgE levels in their umbilical cord blood were around 66% more likely to have a diagnosis of asthma by the age of 10.
The researchers also found that children who showed no signs at all of an increased sensitivity to allergens in their early years were three times more likely to develop asthma if they had high levels of the antibodies in their umbilical cord blood.
This apparent paradox can occur because in some cases of asthma the immune system triggers airway inflammation without any specific reaction to a known allergen.
Immunity starts early
The researchers say dramatic changes in the immune system occur during pregnancy - and the immune response of the foetus - to make IgE antibodies - can start as early as the 11th week.
This can be influenced by the mother's environment as the same factors that impact on her may reach the baby via the placenta.
They suggest their findings indicate that foetal immune system programming could be more important than what happens after birth in influencing the development of childhood allergies.
Lead researcher Dr Hasan Arshad told BBC News Online: "Over the last 10-15 years a lot of research has focussed on the early life influences that may contribute to the development of asthma in a child, but it may be that we have to go back even further and look at the pregnant woman's environment."
However, Dr Seif Shaheen, of King's College London and Asthma UK Research Fellow, said it would be wrong to overlook the impact of exposure to allergens following birth.
"Whilst there is an increasing body of evidence that the environment in the womb may influence the development of atopy (allergy) and asthma, it is also likely that the environment once the child is born plays an important role too.
"We are all born, to a lesser or greater extent, with our immune system skewed towards an allergic type response.
"Environmental exposures after birth are likely to determine whether this allergic tendency carries on through childhood or disappears."
Professor Andrew Peacock, of the British Thoracic Society, said: "This research indicates that umbilical cord blood may hold valuable clues as to why one in five children in the UK have asthma and one in three adults are developing an allergy.
"More research needs to be done in this area if we are to have any chance of turning round the four fold increase in allergic conditions that we have seen take place in the UK over the last 20 years."