LONDON - The development of allergies in children may be related to the age at which their mothers started having menstrual periods, new research suggests.
The prevalence of allergies and asthma has increased by as much as 80 percent in the developed world over the last decade or so, particularly in children, and scientists can't explain most of the increase. On average, 20 to 25 percent of people in the United States and elsewhere in the developing world have asthma or some other allergic disorder.
A study published this week in the British medical journal Thorax found that allergies were more common among people whose mothers started menstruating early and less prevalent among those whose mothers had their first period later, particularly after age 15. It is the first time such a link has been made.
The younger their mothers were when they had their first period, or menarche, the more likely the subjects were to have allergies as adults.
Dr. Baizhuang Xu, who led the study at Imperial College, London University, said his preliminary findings support the idea that the female sex hormone estrogen may play a role in allergies. Puberty is triggered in girls by an increase in estrogen and experts have noted that girls are reaching puberty earlier these days.
Previous studies have suggested that women with high estrogen concentrations may be more likely to suffer allergies and that conditions in the womb might influence the development of allergies later in life.
Lindsay Forbes, an asthma and public health researcher at Guy's Hospital in London, said the findings were intriguing, but warned they were preliminary.
''It may be causal, it may not be. There may be some environmental factor that is common to both allergies and early menarche,'' she said. ''But it's one of the possibilities.''
She said if the connection turns out to be real, while nothing can be done to change the onset of menstrual periods, other estrogen-related influences such as hormone replacement therapy and the contraceptive pill can be controlled.
The findings are encouraging for further research investigating a possible link between estrogen and allergies, she said.
Xu noted indirect evidence suggesting the hormone might have a role.
Dr. Fernando Martinez, director of the Respiratory Sciences Center at the University of Arizona, said while the findings were plausible they do not take into account the fact that first-born children are more likely to develop asthma and allergies. The study does not distinguish between first-born and later-born children.
Allergies and asthma are more prevalent in young boys than girls, but by puberty, the opposite is true.
Women have a higher incidence of allergies than men until the age of about 40 or 50, when menopause sets in. The prevalence of allergies is balanced equally between men and women after that age, except for women who are taking hormone replacement therapy, Xu noted.
Some experts believe the age of onset of menstrual periods could be an indicator of a woman's estrogen levels. The earlier she starts menstruating, the higher her estrogen levels are in adulthood.
Estrogen can also affect the functioning of the immune system.
Xu's study involved 5,188 people born in Finland in 1966. In 1997, at the age of 31, they were given a skin prick test to determine whether they had allergies. Their medical records included the time they were in the womb and showed at what age their mothers had their first periods.
Overall, 33 percent of the men and 28 percent of the women had allergies. The average for the general Finnish population is about 30 percent.
A total of 35 percent of those whose mothers started menstruating at 12 or younger had allergies. For those whose mothers' first period was at 13, the rate was 32.8 percent. It was 30.2 percent for children of women who reached puberty at 14 and 30 percent for those with mothers whose first period came at 15.
Only 26.4 percent of people whose mothers started menstruating at 16 or later had allergies.
That means those whose mothers had their first period at 12 or younger were 1.5 times more likely to suffer allergies than children of women who began menstruating later.
Xu acknowledged the association is not strong, and said research needs to be done in other countries, and with a direct measurement of estrogen instead of using menstrual periods as an indicator.
''It is possible that differences in the maternal estrogen environment, represented by varying age at menarche, could program the immune system of the fetus in a manner that could affect the (allergy) status later in life,'' the study said.