First day of your last period The most common way to calculate your pregnancy due date is by counting 40 weeks from the first day of your last menstrual period (LMP). And thats how most healthcare providers do it.
The most common way to calculate your due date is to start with the first day of your last menstrual period (LMP). Add 7 days, and then count backward 3 months. For example, if your last period started on March 20, you would add 7 days to get March 27. Then subtract 3 months to get a due date of December 27.
How can you find out your due date without knowing your last period?
What if I dont know the date of my last menstrual period?If you know you had your LMP during a particular week, your doctor can estimate your due date accordingly.If you have no idea when your last period was, your doctor may order an ultrasound to determine your due date.