The average pregnancy is said to last 265 days (or 38 weeks) from the time of conception. As it can be difficult to pinpoint the exact time of conception, the date that is most commonly used to determine the Estimated Date of Delivery (EDD) is the first day of the last menstrual period, known as the LMP. Because the menstrual cycle averages 28 days, with ovulation (and conception, if it takes place) occurring about 14 days into the cycle, the EDD is determined by adding 7 days and nine months (or 40 weeks) to the LMP (i.e. adding two weeks to the theoretical date of conception). If the exact date of conception is known – for example, when the pregnancy has been achieved through fertility treatment – the EDD can be calculated by adding 38 weeks to the date of conception.