Understanding Your Due Date: Navigating Discrepancies & Ultrasound Dates

Question
Hello Brenda,
My question is regarding my due date. This is my second pregnancy and my LMP was Sept 20/08 putting my Due date @ May 27/09. I have had four u/s (from three different Tech's)and been told three different dates. My last u/s was this week which another Technician had stated I was 20wks and four days and puts the due date to July 30/09. This being another vast difference in a date change. He stated the baby is 1lb and all measurements add up perfectly. Which I am happy to have that date as long as the baby is doing well. I have not got a specific due date from my GP either and am just trying to get my materinty leave set up. Therefore I need to know what is my actual due date is. As you well know after 28wks I will be sent to an OB. What do you think is the reason for these date changes? And do you think I should wait for the OB to decide what he thinks is the date and than go from there?

Thanks Chi

Answer
Dear Chi,

WOW!!! That is too much of a difference in due dates to be a difference in the skill of the ultrasound technician! Also - 28 weeks is rather late to wait to see an OB for a pregnancy. By 28 weeks many screening tests will often have already been performed. Your regular OB doctor will give you a schedule of all the doctor's visits you should have while pregnant, but most experts suggest you see your doctor:
- about once each month for weeks 4 through 28
- twice a month for weeks 28 through 36
- weekly for weeks 36 to birth
If you are older than 35 or your pregnancy is high risk, you'll probably see your doctor more often.

As far as the ultrasound dating -- In general, ultrasonography dating of a pregnancy is more accurate than dating using the date of the mother's last menstrual period. Now, you didn't say when you had the other ultrasounds done or what the results were, but the biometric crown-rump measurement (CRL) of the six to 12-week fetus provides the most accurate measurement - within three days of true gestational age. From 12 to 18 weeks gestation, other anatomic structures are typically measured and biometric measurements are usually accurate to within a week of gestational age. After 26 weeks gestation, fetal variability has greatly increased and the dating provided by biometric measurement is less accurate, but the method is still used. By late in the third trimester, measurements are accurate only within a two-to three-week window.

I hope this information has helped you and answered your question. I wish you well.

Brenda