By Caroline Ryan
BBC News, Prague
Giving birth to a boy can increase the risk some women will have recurrent miscarriages, researchers have found.
Boys seem more likely to trigger the immune system
University College Hospital Copenhagen looked at over 300 women who had had a healthy child, but then miscarried in their next three pregnancies.
They found those whose first baby was a boy had a third of the chance of having another baby as those who had a girl.
A boy seems to trigger an unnatural immune response in some women, they told a European fertility conference.
Pregnancies often fail because a foetus would not survive, perhaps because it has a chromosomal abnormality.
If a woman has three miscarriages in a row, she is referred to a specialist for help.
One per cent of all women experience such recurrent miscarriages - and a third of those have already had a baby.
These women presume they do not have a problem becoming pregnant, and seek to have another baby.
Shock to the system
The team at the Copenhagen clinic looked at 305 women who already had a child.
One hundred and eighty four - 60% of them had had boys.
The usual rate in a population would be 51%.
The researchers calculated women whose first child was a boy had a third of the chance as those who had had a girl of having a second baby, even after treatment.
And those women who do have second babies are more likely to have girls.
Dr Henrietta Svarre Nielsen, who led the research, said: "It is known that when a female carries a male, it is strange to her immune system.
"And up to 22 years later, you can pick up cells in her immune system which act against males.
"Carrying a male baby is normally tolerated - but in some cases, that obviously goes wrong."
She said her research was at an early stage, so had not identified what the problem might be.
But Dr Svarre Nielsen said it might be that the woman¿s body has an unusual immune reaction to the presence of male cells, which then worsens so it is triggered even by being pregnant.
She added: "The fact that we can now detect a defect might give us a clue to what happens in a normal pregnancy."