03/07/2017 05:54 BST | Updated 03/07/2017 06:56 BST

Father's Age Affects Fertility, Harvard Study Reveals

'Tick-tock. Tick-tock.'

'When are you planning on having kids?'

'Don't leave it too late.'

These are phrases that every 30-something-year-old woman who is yet to push a small human out of her uterus has no doubt heard time and time again, but men are generally given a free pass when it comes to putting off fatherhood.

Hollywood celebrities like Charlie Chaplain -- who fathered his eleventh child at the grand old age of 73 -- and Robert DeNiro -- who welcomed a daughter in 2011 at the age of 68 -- reinforce the idea that men can conceive a child at any age.

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Hollywood star Steve Martin became a first-time dad at 68, but not all men can conceive children later in life.

But new research out of Harvard Medical School suggests men's fertility declines with age, just like women's.

It's not as drastic, but it's still significant enough to have a serious effect on a couple's ability to have children.

IVF Success Rates Fall For Older Dads

The study analysed the success rates of 19,000 in vitro fertilisations (IVF) performed in 7,753 couples over a four year period.

"Generally, we saw no significant decline in cumulative live birth when women had a male partner the same age or younger," researcher Dr Laura Dodge from Beth Israel Deaconess Medical Center and Harvard Medical School explained.

But women with older male partners had a harder time having a baby through IVF than women with partners of the same age or younger, the study found.

For women in their 20s, almost three quarters (73 percent) of IVF cycles were successful if their partner was in their early 30s, but that fell to less than half for those with partners between the ages of 40 and 42.

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Children of older dads are also at higher risk of psychiatric disorders, including autism.

For women in their late 30s -- a time when female fertility typically starts to significantly decline -- the age of their partner also had a huge effect. For women aged 30 to 35 with partners in their 20s, the success rate was 70 percent. This fell to 54 percent for women with partners aged 30 to 35.

Dr Dodge will present the findings at the annual meeting of the European Society of Human Reproduction and Embryology on Tuesday, July 3.

What's Behind Male Infertility?

The focus on women's biological clocks has meant the effect of men's age on fertility has been largely overlooked -- despite the fact infertility affects as many men as women.

From the age of 40, men's quality and quantity of sperm decline and for every Charlie Chaplin out there, there are many more men who are unable to father a child later in life. It can take five times as long for men over 45 to conceive, and the chance of miscarriage doubles compared to a man under 25.

Numerous studies have also linked older fathers with increased risk of psychiatric disorders, including autism and schizophrenia.

The best preconception advice we can offer is to maintain a healthy lifestyle."

Women are born with a set number of eggs, which accumulate mutations as they age, but the reasons behind declining male fertility are not as well understood.

Dr Dodge said that reduced sperm quality and quantity, increased damage to the sperm's DNA, and lifestyle factors could all play a role.

So is there anything older would-be fathers can do to increase their chances of having a baby?

"It's hard to say without knowing the precise mechanisms involved," said Dr Dodge.

"Most preconception advice for men focuses on semen quality, though studies suggest that this likely cannot fully ameliorate the effects of male reproductive ageing. So in the absence of clear evidence of the mechanisms, the best preconception advice we can offer is to maintain a healthy lifestyle."

So go forth, men, and eat your vegetables -- it just might help you become a dad.