How important is the due date

Most doctors refer to the due date as EDD or Estimated Date of Delivery.  And EDD are just that, estimated dates, not exact dates.  What this means is that due dates are not sacred, even if your periods are regular.  But due dates are helpful for the expectant mother to prepare herself for childbirth and also to plan for the new arrival in the family. An estimated due date can certainly help in planning for all of the following:

• Physical exercise for the mother (muscle-toning for childbirth);
• Mental preparation (possible change in lifestyle);
• Financial adjustments (babies cost money, even before they reach college!);
• Maternity leave planning (giving your company a notice period); and
• Most fun of all – baby shopping. 

I have heard that very few women actually deliver on their due date

True.  Less than 50% of expectant mothers deliver on their expected due date.  If your periods come at a regular interval of 28 days, you are more likely to deliver close to your estimated due date.   If your cycles are longer than 28 days, you are more likely to deliver later than your EDD, and if they are shorter, earlier. However, if you have irregular cycles, then this dating system may not work; a sonogram taken later in the pregnancy may be a better indicator.