A comprehensive study has found a woman's chance of giving birth to a baby within five years of starting fertility treatment is at 70 per cent.

Two thirds of women under 40 starting fertility treatment will have a baby within five years, a large-scale Danish study has found.

There is continuing debate on how to best measure the "success" of fertility treatment, but the authors of a large study in Denmark claim they've been able to provide couples with a "reliable" long-term predictor of success - based on years, not on IVF treatment cycles.

The comprehensive study, led by Dr Sara Malchau of Copenhagen University Hospital, analysed the birth records of more than 20,000 women who had fertility treatments in Denmark between 2007 and 2010.

Results showed that after two years 57 per cent of women had a baby through the use of either IVF or intrauterine insemination (IUI).

Total births increased to 65 per cent after three years, and to 71 per cent after five years.

Further analysis showed age was the greatest determinant of success.

At five years, total birth rates were 80 per cent for women under 35 years, 60.5 per cent for those aged 35-40, and 26 per cent for those aged 40 and over.

Overall chances of a live birth are good, but the point that the study makes is that successful treatment takes time, says Dr Gareth Weston, acting medical director at Monash IVF Group.

He says it's important for any couple to be properly assessed to determine whether they actually need IVF in the first place.

Many problems will be able to be corrected before the use of reproductive assistance, such as removing endometriosis in women or a cyst blocking a fallopian tube.

He urges couples not to rush into IVF and to exhaust all other options first, such as IUI.

One way or another, women under 40 willing to persist with treatment will achieve a baby in the vast majority of cases, Dr Weston said.

"I always say to my patients it's a marathon and not a sprint."

However the giving of accurate information is paramount for women over 40, he emphasised.

"As they approach 45 from 40 the success rates do drop off significantly and that is the group which needs to have very careful and accurate information about the chances of success."

Criticism has been levelled at the IVF industry in the past about the accuracy of information available.

If an IVF clinic is not willing to reveal their success rates before the start of treatment then that service should not be used, advises Dr Weston.