The important thing to keep in mind when thinking about the accuracy of your due date is that, truly, it is just a best guess.  Statistically speaking, most women do not have their baby on their due date.  While an average pregnancy is 40 weeks, the fact of the matter is that a normal healthy pregnancy may last anywhere from 37 to 42 weeks.  Many babies are born early, and many babies are born after.

The due date is calculated by looking at the start of your last menstrual period, also called the LMP.  The average pregnancy will last 280 days (40 weeks) after the start of the last menstrual period.  Of course, this assumes that your period is always regular and always 28 days long.  If you have a longer cycle than 28 days, you are likely to deliver after your due date.  If you normally have a shorter cycle, you are more likely to deliver before your due date.

Your health care provider may also try to gauge your due date during your first trimester through the use of an ultrasound.  During an ultrasound, the ultrasound technician will measure various parts of your baby.  Your health care provider might choose to move your due date based on these measurements, if they don’t match up with the due date calculated by your last menstrual period.

Even if you have a regular cycle and an ultrasound confirms your due date, the fact of the matter is that your baby might not be born on your due date.  In fact, researchers aren’t entirely sure what exactly it is that triggers a woman’s body to go into labor.  It may be that your baby is anxious to get out a week or two early, or maybe he wants to stay in his comfy home a little bit longer.

Regardless, once you have reached 40 weeks of pregnancy, your health care provider will probably want to begin regular stress testing to make sure that your baby is safe in the womb.  If your health care provider sees that your baby is in distress, it may be that she will suggest that labor be induced.

How Accurate is my Due Date?