At OvuFriend, the first day of your last menstruation is considered to be the start of your pregnancy. This is a commonly accepted method of calculating pregnancy length. Your doctor will probably determine the beginning of your pregnancy this way during your first visit. He or she will ask you when you last menstruated and will note that date as the first day of your pregnancy.
Because it's often difficult to pinpoint the exact moment of conception (ovulation), doctors calculate the length of the pregnancy as 40 weeks (an average pregnancy lasts 38 weeks long) and start to count it from the beginning of the first day of the current cycle (they assume that ovulation falls, on average, on the 14th day of the cycle).