We are taught that ovulation occurs fourteen days before our period begins, regardless of cycle length. New research from the US challenges this assumption

The gospel according to the infertility industry is that we ovulate 14 days before our period begins - irrespective of the length of cycle. Not so, says Alan J Wilcox, MD, head researcher at the National Institute of Environmental Health Sciences (NIEHS) in Durham, North Carolina.

When we are looking for our fertile period - either while practising natural family planning or trying to conceive - we base our calculations on six days bang in the middle of our period. The study, which appeared in the British Medical Journal, found astonishingly, that some women are potentially fertile on every day of the menstrual cycle.

When we are young the standard advice is, never take a risk on any day - including during our periods. This is a kind of catch-all to prevent youthful mistakes. As we grow older, we are told there is a 'safe' period for unprotected sex. Now it seems the more cautious advice is actually more accurate.

The 213 women in the study produced 30,000 urine samples and provided diaries of when and how often they had sex. The results, not unnaturally, took several years to analyse. Earliest ovulation in the study occurred on Day 8 (if Day 1 is the first day of bleeding) the latest on Day 60. There are many other variables at work in achieving a pregnancy, such as viability of sperm and receptivity of the uterus.

Wilcox concludes that the best way to get pregnant is to let nature take its course: 'Most couples will be pregnant within three to four cycles' he says. But he is at pains to point out that women with regular cycles who rely solely on the rhythm method may be in for an unexpected surprise.