Men should start a family before they age of 35 to avoid risk to their unborn babies, researchers claim.

A study tracking 40million babies found the risk of birth complications start to increase when fathers are in their mid-30s, and significantly rise from the age of 45.

For decades women have been told they will put their children at risk if they pursue a career first and leave it until they are older to have a family.

But the new study, published in the British Medical Journal, shows men should also take responsibility.

Researcher Professor Michael Eisenberg, of Stanford University School of Medicine, said: 'We tend to look at maternal factors in evaluating associated birth risks.

'But this study shows that having a healthy baby is a team sport, and the father's age contributes to the baby's health, too.'

He said once a father hits 35, there's a slight increase in birth defects, but the risk increases more sharply as men age into their 40s and 50s.

This is because with every year that a man ages, he accumulates on two new mutations in the DNA of his sperm.

Compared with fathers between the ages of 25 and 34, infants born to men between the age of 35 and 44 were about five per cent more likely to be born premature or of low birth weight.

For men aged 45 or older they were 14 per cent more likely to be admitted to intensive care, 14 per cent more likely to be born prematurely, 18 per cent more likely to have seizures and 14 per cent more likely to have a low birth weight.

If a father was 50 or older, the likelihood that their infant would need ventilation upon birth increased by 10 per cent, and the odds that they would require intensive care increased by 28 per cent.

Professor Eisenberg added: 'What was really surprising was that there seemed to be an association between advanced paternal age and the chance that the mother would develop diabetes during pregnancy.'

For men age 45 and older, their partners were 28 per cent more likely to develop gestational diabetes, compared with fathers between 25 and 34.

He said the possible biological mechanisms at play on this point are unclear, but he suspects that the mother's placenta has a role.

Some 18 per cent of children born in England and Wales have a father aged 40 and above, according to the Office of National Statistics.

The average age of fathers in 2016 was 33.3, nearly four years older than 1974, when they were 29.4.

The age of mothers have increased at almost exactly the same rate – they had an average age of 30.4 in 2016, and 26.4 in 1974, also a rise of four years. But Professor Eisenberg said the absolute risks are still relatively low.

He compared the increased risks to buying lottery tickets. 'If you buy two lottery tickets instead of one, your chances of winning double, so it's increased by 100 per cent,' he said.

'But that's a relative increase. Because your chance of winning the lottery started very small, it's still unlikely that you're going to win the lottery. This is a very extreme example, but the same concept can be applied to how you think about these birth risks.' The researchers wrote: '

A significant number of these negative birth outcomes were estimated to be prevented if older fathers had elected to have children before the age of 45 years.

'The risks associated with advancing paternal age should be included in discussions regarding family planning and reproductive counselling.'

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