'No such thing as safe time for sex'

New research indicates there is no "safe" time to have sex for millions of women using natural birth control methods.

Extraordinary new findings appear to show that some women may ovulate up to three times a month - rather than just once as previously thought.

If true, the discovery will overturn more than 50 years of medical orthodoxy and could signal the end of natural birth control. It may also lead to more effective forms of contraceptive pills and radical changes in fertility treatments such as IVF.

The man leading the Canadian study said today he was "flabbergasted" by the results, adding: "It may be the case that for a large proportion of the female population, natural birth control is simply not an option."

However, the findings are likely to be greeted with heavy scepticism from the medical and scientific community, who will want to see more evidence. The vast majority of experts believe women only ovulate once a month.

The new research will be published this week in the journal Fertility and Sterility. Previously, it was believed around a dozen egg follicles grew during the menstrual cycle. From these, it was thought only one egg was released each month. This knowledge was based on blood samples and menstrual diaries taken from women.

However, the new study involved taking high resolution ultrasound scans of more than 60 women every day so that each follicle could be tracked.

It found that women experience two to three hormone surges each month, each with the potential to produce an egg. A total of 10 per cent of the women in the study were found to produce more than one egg in each cycle.

Dr Roger Pierson, the director of the Reproductive Biology Research Unit at the University of Saskatchewan in Saskatoon who led the study, said: "It appears nobody actually carried out a detailed analysis before. We are literally going to have to re-write medical textbooks."

While he stressed that the research was still at an early stage, he believes it could have monumental effects for both contraception and fertility treatments.

He said: "We all know people trying to use natural family planning, and we have a word for those people. We call them parents." If true, the discovery could also have a big impact on the development of oral contraceptives, as scientists will now have a much better idea of exactly what happens throughout the menstrual cycle.

Dr Pierson claims the research should also lead to improvements in IVF treatment.