The duration of a pregnancy can be calculated using either the date of conception or the date of the last menstrual period (LMP). However, most healthcare providers use LMP as it is easier to track. Pregnancy is typically counted as 40 weeks from the first day of your LMP. This means that if you know your LMP, you can estimate your due date by adding 40 weeks or 280 days. Keep in mind that this method assumes a regular 28-day menstrual cycle and ovulation occurring on day