Behind the headlines: Do sons raise risk of miscarriage?

A male first child may reduce the chance of further births. Rachel Liddle reports.

What is the story?

If a woman's first child is a boy she has an increased risk of miscarriage in future pregnancies, according to media reports.

Reports said women with a male firstborn are only a third as likely to have a second child as those whose first child is female.

They claimed that some women have an immune reaction to genes on the Y chromosome of the male foetus. This overactive immune response leads to the woman's body rejecting embryos in subsequent pregnancies.

What is the research?

Danish researchers looked at live-birth rates among women who had one child but had since suffered from unexplained recurrent miscarriage.

The team studied a total of 305 women referred to a fertility clinic after suffering three or more miscarriages since the birth of their first child.

Among the women, 184 (60 per cent) had a male firstborn. Analysis showed 56 per cent of patients with a firstborn boy had a subsequent live birth after attending the clinic, compared with 78 per cent of those who had a girl.

Looking at live-birth rates among women who fell pregnant after attending the clinic, women who had a son were 37 per cent as likely to have another child as those who had a daughter. This was the case even after confounding factors, such as the number of previous miscarriages, were taken into consideration.

The findings were presented at the annual meeting of the European Society of Reproductive Health and Embryology, held this month in Prague in the Czech Republic.

What do the researchers say?

Lead researcher Dr Henriette Svarre Nielsen, from the Fertility Clinic in Copenhagen, said that the findings were preliminary.

She also pointed out that recurrent miscarriage that cannot be explained by problems such as chromosomal abnormalities in the parents or malformations in the uterus is rare, affecting just 1 per cent of women. Of those, only a third have had one previous successful pregnancy.

'The most interesting thing is that the prognosis in this group of women is dependent on having a first boy rather than a first girl,' said Dr Svarre Nielsen. 'If a patient has had a girl you know the prognosis is better.'

She suggested that the reason women with unexplained secondary recurrent miscarriage may have a lower chance of having a second child if they already have a son is that the woman's immune system is already primed against male tissue.

'It is known from the literature that reactions against male specific tissue types are present in females 22 years after delivery,' she said. The mechanism may lead the mother's body to reject subsequent male foetuses.

Dr Svarre Nielsen added that having a male child may not always be the underlying cause of miscarriage. Other factors could be at play. The chance of having a live birth can be good for women who have experienced two miscarriages since the birth of a child.

'It is important to find out if a patient is suffering unexplained recurrent miscarriage and if so refer her to a specialist clinic,' she said.

What do other experts say?

Professor James Walker, professor of obstetrics and gynaecology at St James' Hospital, Leeds, said: 'It is an interesting explanation for the already known immune causes of miscarriage, but women who have had a male child should not be fearful of miscarriage.

'The level of miscarriage is actually quite low in the group, compared with first-time pregnancy.'

Professor Walker said it was known that antibodies against a male foetus are found at high levels following miscarriage, with levels increasing with the number of miscarriages.

However, he added that the trend observed in the study might also be due to all male foetuses having a lower chance of successfully being carried to term.

'Women are more likely to miscarry a male foetus anyway,' he explained.

Professor Walker said GPs could reassure women who had given birth to a son not to worry about subsequent miscarriage risk. However, any patient who has miscarried three or more times should always be referred to a specialist for treatment.

- European Society of Reproductive Health and Embryology

INFORMING PATIENTS

- Unexplained recurrent miscarriage is rare, affecting less than 1 per cent of women.

- Among such women, having a boy can reduce the chance of having another child by two thirds.

- An overactive immune response by the mother to a male foetus could explain this trend.

- Despite the reduced chance, more than half of secondary recurrent miscarriage patients with a son have another baby.

WHAT THE PAPERS SAID

"Baby boys can increase risk of miscarriage" - the guardian

"Having a son 'can raise risk of miscarriage'" - The Daily Telegraph

"Giving birth to a boy 'can raise the risk of miscarriages later'" - Daily Mail.

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