Expert: Catherine Beier MS CBE - 12/2/2009

Question
I'm getting ready to have my "major" ultrasound. I will be 18 weeks. I have heard a lot of women say that their due date was changed after their ultrasound. Why is this? If they change my due date, does this change the conception date? How accurate are ultrasounds at 18 weeks at dating the baby age? I have had 2 ultrasounds so far and they both showed the same conception date and due date, one at 7 weeks and one at close to 11 weeks. Where those the most accurate? Also is there anything I can do now to prepare myself for natural birth?

Answer
Cody,


Early transvaginal ultrasound between 8-10 weeks is the most accurate way to date a pregnancy. They're accurate within +/- 3 days. Your almost 11 week ultrasound was likely the most accurate.



Ultrasound after the first trimester can be off by up to 2 weeks in either direction, as at that point, individual growth rates take over depending upon genetics, maternal health and nutrition.


So, unless an ultrasound shows more than a 2 week difference after the first trimester, then a due date should not be changed.  In either case, you shouldn't use an ultrasound after the first trimester to determine conception - it's just not accurate enough at that point.


And yes, there is much you can do to prepare yourself for natural birth - more than can be detailed in a single answer.  Here is a series of articles that are a good place to start learning more:  http://www.givingbirthnaturally.com/vaginal-birth.html



Best,
Catherine