One theory is that women expecting boys have more testosterone, which weakens the immune system.

London - Having boys may shorten a woman’s lifespan.

Researchers found that women who had a number of sons lived for an average of eight and a half months less after completing their family than those who only had daughters.

The shorter life expectancy remained regardless of the wealth or social status of the mothers, the study found.

But the gender of the children made no difference to the father’s life expectancy. The researchers said this suggests there is a biological reason why women who have only boys die younger.

One theory is that women expecting boys have more testosterone, which weakens the immune system. Boys also grow faster in the womb and are usually heavier to carry, placing more strain on the body.

The research, led by Dr Samuli Helle of the University of Turku in Finland, was published in the Royal Society journal Biology Letters. The team looked at the post-reproductive survival of more than 11,000 mothers in pre-industrial Finland from parish records kept by the Lutheran Church.

The data was collected from individuals born in eight different Finnish parishes, covering the 17th to 20th centuries, when a mostly-agricultural society did not have access to modern birth-control or medical care.

Dr Helle said that a mother who had six sons would live for a further 32.4 years on average after the birth of her last child, while a woman who had daughters could expect to die 33.1 years after her final labour. He said: “The research shows the more sons you have the lower post-reproductive survival was. Biologically, there is a bigger cost associated with having a boy than a girl, so that is one explanation for the shorter lifespan.”

He said that ‘”ocial and cultural reasons could also play a factor”, such as additional support given to mothers by their daughters.

But Dr Helle suggested that similar effects would be unlikely to be seen in modern British mothers as family sizes are much lower and nutrition and medical care have improved greatly. The phenomenon, however, may still affect mothers in the developing world.

A study by the University of Manchester found that women who delay having children into their 30s may live longer than those who become mothers while young.

The hormone oestrogen – released into the body when a woman becomes pregnant – could help extend lifespan, researchers believe.

Although oestrogen naturally declines in women over 30, large amounts are released during pregnancy. It helps keep skin, hair, bones and blood vessels healthy, and protects against osteoporosis. - Daily Mail