First, there’s the question of when the pregnancy began: some women know the date of ovulation or conception, but typically, due dates are based upon the woman’s last menstrual period, a method which carries much potential for error.Second, there’s a question of how long the pregnancy will last.If you do know the conception date, this is counted as day 14, not day one. If your cycle is irregular or you've recently been on the pill, working out the date from your LMP probably won't work. If the scan date differs from your period dates, the scan date will be used.
This depends on whether you have agreed to have the screening and when the scan takes place. The point of the scan is to work out how many weeks pregnant you are. Ectopic pregnancy and miscarriage: diagnosis and initial management CG 154. You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP). Dating a pregnancy accurately at the earliest possible time is important.In a normal pregnancy it does not matter what the dates are, but if anything should go wrong it becomes very important as to what treatments may be prescribed.