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Nearly three out of four couples that begin fertility treatment will eventually become parents, long-term studies suggest.

The analysis of nearly 20,000 Danish couples found 65% had children within three years and 71% within five years.

Doctors, presenting their data at a fertility conference, said the odds were heavily influenced by age.

But experts said the findings were very encouraging for couples struggling to have babies.

There is strong evidence that about one in three cycles of IVF is successful in women under the age of 35.

But what happens in the long run, when some couples try over and over again, others give up and some have problems that cannot be treated, has been uncertain.

Researchers at the Copenhagen University Hospital used rigorous registry records in Denmark to follow 19,884 women from the moment they started fertility treatment.

The results, presented at the European Society of Human Reproduction and Embryology, showed that more than half had given birth within two years, rising to 71% after five years.

For women under 35, 80% had children within five years. But the figure fell to 61% in those between 35 and 40 years old; and fell again to 26% in women over 40.

'Very good chance'

Dr Sara Malchau, one of the researchers, told the BBC News website: "There is a very good chance of having a child, even if you have difficulties conceiving on your own.

"Most causes of infertility can be overcome, but age is the most important factor to predict if treatments are going to be successful or not.

"Also women with a body mass index under 30 had better outcomes as well as women who didn't smoke."

The study also found that nearly a fifth of the women under 35 ended up conceiving as a result of sex - despite having sought fertility treatment.

However, Dr Malchau cautioned Denmark prioritised fertility treatments that made it easier for women to keep trying. Many other countries are less generous.

Common causes of infertility

Women:

damage to the fallopian tubes

ovulatory problems

endometriosis

age

polycystic ovary syndrome

diabetes

being overweight or underweight

smoking

Men:

low sperm count

problems with the tubes carrying sperm

problems getting an erection or ejaculating

diabetes

being overweight

Source: HFEA

Prof Nick Macklon, from the University of Southampton, said: "It really does provide some encouraging news for those who are about to embark on the journey of fertility treatment - the chance of having a baby is good.

"There will always be individual factors that affect an individual's prognosis, but overall it shows us fertility treatments are working.

"There's been a lot of debate about whether women should be having their children earlier, most of the people in my field would be of the view that if you can start earlier then your chances of completing the family you desire is going to be much higher."

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