Pregnancy lasts an average of 280 days or 40 weeks (almost 10 months) from the first day of your last menstrual period (LMP). The first day of your last menstrual period is considered day one of pregnancy, even though you probably didn't conceive until about two weeks later. (Fetal development lags two weeks behind your pregnancy dates.)

Calculating your due date is not an exact science. Very few women (about 4%) actually deliver on their due date, so while it is important to have an idea of when to expect delivery, try not to get too attached to the exact date.

How Can I Calculate My Due Date?

If you have regular menstrual cycles, there are two ways to calculate your due date (otherwise known as the EDC-your estimated date of confinement).

Naegels Rule

Naegels rule involves a simple calculation: add seven days to the first day of your LMP, then subtract three months.

For example, if your LMP was November 1, 2008:

1. add seven days (November 8, 2008);
2. subtract three months (August 8, 2008); and
3. change the year if necessary (to the year 2009 in this case).

The EDC would be August 8, 2009.

Pregnancy Wheel

The other way to calculate your due date is to use a pregnancy wheel. This is the method that most health care providers use. If you have access to a wheel, it is very easy to estimate your due date. You or your provider can locate the date of your last menstrual period; when you line up the date with the indicator, the wheel will display your due date (accurate within a couple of days). Please note that the wheel is designed for use by women with regular 28-day menstrual cycles.

One person may tell you your due date is August 8 and another may say August 9. Remember-the due date is only an estimate of when you will deliver. The chances of actually delivering on this date are very slim.

What If I Don't Know My LMP?

This is more common than you would think. If you can't remember the first day of your last menstrual period there are other ways of figuring out your due date. If you think you know when your period was (within a week), your doctor will probably estimate your due date according to this date as long as other evidence (physical exam or ultrasound studies) doesn't contradict it. If you have no idea when your last period was, your doctor may order an ultrasound to determine your due date.

Source: Heathline