Around three-quarters of women who start fertility treatment will have a baby within five years, a large study has found.

According to Danish researchers, these babies will be born as a result of both fertility treatment and natural conception.

The study involved an analysis of the birth records of over 19,800 women who underwent fertility treatment in Denmark between 2007 and 2010. It found that 57% had their baby as a result of fertility treatment, while 14% conceived naturally.

Of the 57% of women who had their children as a result of fertility treatment, 46% conceived using IVF (in vitro fertilisation) when IVF was the first fertility treatment.

Another 34% conceived as a result of IUI (intrauterine insemination), when it was the first fertility treatment.

Some 16% of women who started treatment with IUI had a baby after five years, not as a result of the treatment, but after a spontaneous conception.

The study noted that over half (57%) of the women gave birth within two years of beginning fertility treatment.

As expected, age was seen as the greatest determinant of success. At five years, the total birth rates were 80% for those under the age of 35, 60% for those aged 35-40 and 26% for those aged 40 and older.

"Infertility patients have two key questions -what are our chances of having a baby, and when will it happen. These results help us provide realistic information based on their age and chance of natural conception.

"Overall, chances of a live birth are good, but successful treatment takes time. Couples will often need several treatment cycles. And even though the greatest chance of conception is following treatment, there is still a reasonable chance of spontaneous conception," commented Dr Sara Malchau of Copenhagen University Hospital.

She pointed out that spontaneous conceptions both after or between fertility treatments are seen at all ages and irrespective of the types of infertility, however they are most common in women under the age of 35 who are starting IUI.

After five years of treatment, 18% of those under the age of 35 attempting IUI had given birth after a spontaneous conception compared to 8% of women over the age of 35 who started with IVF.

Dr Malchau believes that these results are robust and realistic and offer a long-term prognosis for people starting fertility treatment.

"At this point, couples have no idea how many treatment cycles they will need or have, so a prognosis based on fixed points in time better reflects their prospect of conception and delivery than birth rates after different numbers of attempts," she commented.

Details of these findings were presented at the Annual Meeting of the European Society of Human Reproduction and Embryology in Helsinki, Finland.