Ultrasound Accuracy: Understanding Gestational Age & Dating

Question
QUESTION: Hi How accurate is a 10 wk ultrasound? If the ultrasound said i was 9 weeks 5 days and my conception was october 31 2008 and due august 5th 2009.. Is that ultrasound pretty accurate? when i had asked tis on another site they keep saying i must have made a mistake on the dates but no I have the paper right here in front of me from my ultrasound

ANSWER: With an ultrasound, they determine how many weeks you are counting from 2 weeks BEFORE conception, usually from a woman's LMP.

So if at 9 weeks 5 days they said you were due August 5 and you counted back 9 weeks and 5 days from when you had that ultrasound, you would come up with a date in late October.

However, you didn't actually get pregnant until mid-November, probably somewhere between November 10-15.

If you had actually conceived October 31, you would expect to see a due day of around July 24.

I hope that helps!

Dorothy H, LCCE

---------- FOLLOW-UP ----------

QUESTION: See this is where everyone gets confused the ultrasound ALSO said I concieved october 31st.. with this everyone gets confused and says no you didnt concieve then but why would the ultasound say i CONCIEVED that date even with the ultrasound having the lmp as november 2nd.. Maybe the november 2nd wasnt a period so thats why the ultasound says october 31st i concieved so this is where the confusin lies.. Please help me understand this thanks

Answer
I have no idea why they they would say that the conception date was Oct 31, especially since your LMP was Nov 2 AND they put your due date as August 5.

Something isn't adding up here.  I'm guessing that if they wrote Conception: October 31, that it was shorthand for "This is the date we're using to determine conception rather than using the LMP of November 2" and not that they're saying that conception actually occurred that day.

Even though it's a couple of years later, you can always call the place where you got the ultrasound done and ask. Hopefully they can clear it up for you, because you're right, it doesn't make sense.

Dorothy H, LCCE