dating a pregnancy

  I am currently pregnant with my third child.  My first was born right on the due date according to my last period.  (There was no ultrasound done)  My second was born twelve days late according to my last period but a week early by the ultrasound done around 22 weeks.  Now, with my third pregnancy, I once again have two different due dates--eleven days apart (the ultrasound done around 20 weeks being the later date).  How can there be such a discrepency when my periods have always been quite regualar?  My doctor is having a hard time dating my pregnancy because I never get very big  (My babies have been close to 8 pounds).  He says he wouldn't be surprised if I go over the latest due date even.  Could birth control pills taken the month before I became pregnant affect my cycle?
Dear Tiffany:
A last menstrual period is only accurate to date pregnancy is (1) your cycle is regular and repeats every 28 days; (2) the last period was spontaneous (you did this yourself) and not the result of drug therapy. For all other persons, a last menstrual period does not reliably predict the date of delivery.
An EDC (estimated date of delivery), whether done on a wheel calendar that obstetrician carry in their pocket, a calculator/computer program, or an ultrasound examination, is an ESTIMATE. All these calculations have an error of plus/minus two weeks. Thus, when your EDC calculates as June 1, we are saying you will delivery between May 17 (two weeks early) and June 15 (two weeks late).
Only 10% of ladies actually deliver on their calculated EDC.
A menstrual cycle following birth control pill use is not accurate to calculate a delivery date. In your situation, I would rely more on the ultrasound date.
This material is provided for general information purposes only and should not be considered a formal medical evaluation. If you have specific questions, please contact your primary physician.