A new study has revealed that two-thirds of couples undergoing in vitro fertilization treatment need up to six attempts to have a successful pregnancy.

Scientists at the universities of Bristol and Glasgow said the process is not a " single shot" as chances of success increased with the number of treatments, the Independent reported.

The authors found that in all women, the live-birth rate for the first cycle was 29.5 per cent, and remained above 20 per cent up to and including the fourth cycle for women under 40.

The cumulative percentage of live-births across all cycles continued to increase up to the ninth cycle, with 65 per cent of women achieving a live birth by the sixth cycle.

IVF is commonly stopped after three or four unsuccessful embryo transfers, with three unsuccessful transfers labelled as repeat implantation failure.

Professor Lawlor said that these findings support the efficacy of extending the number of IVFcycles beyond three or four.

The study is published in The American Medical Association.