Forget everything you thought you knew about the menstrual cycle.

Well, not everything. That would be excessive. Simply consider some things that you thought you knew to be correct... and reconsider whether they actually are. Because a new study would suggest that a lot of what we think we know about the menstrual cycle is, in fact, wrong. Most, if not all, of us grew up with the knowledge that the 'normal' menstrual cycle was 28 days long. The start of the cycle was your period, smack bang in the middle of the cycle (usually days 12- 14) was when you started ovulating, and the end was when your body realised you weren't pregnant and decided to break down the lining in your womb and make you start bleeding all over again. Sure, most people knew that the menstrual cycle tended to vary in length depending on the individual person, but the hard line from science books, the internet, and most doctors was that the average cycle lasted 28 days. Except, it doesn't. New research conducted by Natural Cycle and UCL found that just 13 percent of people have a menstrual cycle that is 28 days long. Rather, the vast majority of women actually have a cycle ranging from 22 to 30 days, with the average being 29.4 days. And yeah sure, the difference between 28 and 29.4 may not seem like a whole lot, but when you've been told your entire life that 28 days is the norm, it's worth noting that it's actually not. The research considered the menstrual cycles of 60,000 women from the UK, the US, and Sweden, making it one of - if not the - biggest study that has ever been conducted on periods. It proved that if the average 28 day model was incorrect, then assuming that ovulation begins on day 14 is also inaccurate too. This poses an issue for those who may risk having sex during a time of the month they deem to be 'safe' - and for women who are trying to calculate when best to get pregnant. "It is a common belief that ovulation occurs on day 14 of the cycle, but our analysis has shown that for the majority of women in the real-world that this is not the case," reads the study. "These findings demonstrate that the widely held belief that ovulation occurs consistently on day 14 of the cycle is not correct. "Clinically, it is important that women who wish to plan a pregnancy are having intercourse on their fertile days. In order to identify the fertile period it is important to track physiological parameters such as BBT (daily sublingual measurements) and not just cycle length." Basically, you can't be relying on your maths skills anymore. So don't.