Whether you want to have a girl or a boy as your first born, it is safer to carry a female fetus, rather than a male one. At least this is what the medical team of a Copenhagen fertility clinic stated yesterday, at the annual meeting of the European Society of Human Reproduction and Embryology in Prague.

Normally, only 1% of women are facing recurrent miscarriages - three or more - and the most common and efficient treatment for this kind of disorder relates to a blood product that inhibits the immune system and forces it to allow the carriage of the pregnancy.

Sensing that the number of treated women with miscarriages was higher when it involved a first baby boy pregnancy and birth, doctors at the Denmark clinic started to investigate the matter. They came to the conclusion that in the case of women who had previously given birth to a child - and are not at their first pregnancy - the chances to be pregnant again are reduced by nearly two thirds for those who carried a baby boy in their uterus.

It seems that out of the 305 women with unexplained recurrent problems of miscarriage treated at the Danish clinic, 60% had a first boy-child.

The doctors also noticed that the treatment was more efficient for the women previously carrying a female fetus than for those with a male one. Of those who have been successfully treated and could eventually carry a pregnancy, 78% had a girl as their first born and only 56% had a baby boy.

In the opinion of the medical staff conducting the study, male immune system cells can develop an adverse reaction in a woman's body that makes it difficult to have another baby. The male cells remain in a female body for more than 20 years after she has given birth to a male child.

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."