Calculating pregnancy due dates can be quite confusing.

There are several different possibilities. But for the sake of generalization, I will calculate the weeks of pregnancy by using the last menstrual period or LMP as day one. Most pregnancy wheels (the little cardboard wheels you see doctors and nurses use) calculate this way too. But there are several things to keep in mind about how to determine pregnancy due date.

• Pregnancy wheels assume that you ovulate on day 14 of your cycle.
• You can ovulate as early as day 8 and as late as, well there is no definitive answer to how late.
• To get a true date, you can go by conception day, if you know it.

Here's a quick overview of calculating the due date.

If you cycle is basic with ovulation day around day 14, this will be fairly accurate.

Take the date of your LMP (ex: May 21, 2007) and subtract 3 months (ex: February 21) - then take that date (ex: February 21) and add 7 days (ex: February 28) Add the appropriate year - (ex: due date February 28, 2008 for a woman with an LMP of May 21, 2007)

If you wish to use your conception date (if it's not around day 14) then take the day of conception (ex: day 21 of your cycle) subtract 14 days from that date - then do the above calculation - you'll come closer to the actual due date that way!

Baby's will come when they are ready.  Due dates are just a guesstimation as to their arrival.  Ultrasound measurements and actual conception date are things to keep in mind when calculating the due date.

Most importantly is to remember that every pregnancy is different. Due dates are a general time that the baby may come. It's perfectly normal for a baby to arrive 2 weeks prior or 2 weeks after the established due date. Before or beyond that is considered either pre-term or post-term.