Expert: Catherine Beier MS CBE - 11/21/2009

Question
Hi this is Madison again, I wrote you earlier about my pregnancy and rape. I had an ultrasound on 11/14 and they said i was 12w & 2days, i went to one of those 3d places just because i think their machines are newer. I had another ultrasound today 11/20, she said i was 12w & 2 days when I am suppose to be 13w & 1day so it was only a 2 day difference. So with all of these early ultrasounds, rem I had one on 10/8 dating me to be 6 1/2 weeks-7 weeks and then one on 11/3 saying I was 10w & 6 days, do you think i still got pregnant before the rape on 9/9. I'm sorry for reasking, its just a hard time and 6 months is a long time to worry. Thank you very much. Also I know I ask this but some RN's I have talked to have said that early ultrasounds can be off by 1-2 weeks. My ultrasound tech said 3-5 days. That seems more reasonable to me. I can't see how a ultrasound done early on can be 1 week off and i really can't see how it can be 2 weeks off! I thought during the 1st 12 weeks all babies grow at pretty much the same rate. So they're telling me that if i am 12w and 5 days, I could possibly be 11w and 5 days? Or at the worst 10w and 5 days? Or if the machine measure ahead I could be 13w and 5 days or 14w and 5 days? I really just don't see how thats possible so early on. i know 2nd trimesters and third trimesters can be off by a week or two, thats why they give you a 2 week gap either way on your due date but so early on, I can be off by that much? And then they said well it's best to go off your LMP but if you're irregular, you can't do that and get a good age, so i thought that was what the dating ultrasound was for. I have had 4 so far and they all match one another, only off by 2 days. So I can't see how it can be inaccurate and my cycles are irregular so am i just suppose to assume that I could be less or farther along with what my ultrasounds says. I know they do certain test at different times and if you're 2 weeks off either way, you can miss those test or test too early, right. Thank you for clearing up this confusing matter.

Answer
Madison,


First of all, don't go to a vanity 3D ultrasound mill.  Their machines may be newer, but their technicians aren't necessarily skilled nor experienced. Many US states are starting to make these places illegal.


Next, the RNs are wrong.  There is absolutely no way an early ultrasound (especially transvagianl) can be off by 1-2 weeks, for the reasons you stated.  They need to brush up on their training and stop dispensing incorrect information.  The same thing goes for using your LMP.  That is the least accurate way to determine a due date, unless you have a perfectly regular 28 day cycle and always ovulate on day 14.  


Early transvaginal ultrasound has a margin of error of +/- 1 day and early abdominal ultrasound is +/- 3-5 days at most.


Once you're out of the first trimester, genetics, diet, etc take over, growth rates differ and ultrasounds can then be off by 1-2 weeks.


So, I still stand by what I said before.  None of the new information you've provided points to a different conclusion.



Best,
Catherine