Understanding Ultrasound Accuracy for Due Dates & Conception

Question
Hi, I had a light period on June 13-16/17 and then took a pregnancy test on July 13th saying I was pregnant. When I went to the dr on 7/28 I was told that my due date was 3/23/2010 then on the following week on 8/4 they changed my due date to 3/24.The dr said they were using 6/17/09 as my effective last menstrual period because it looks I ovulate late. My question is how accurate is this and would my conception dates then fall between June 24 -July 2 as the online conception calculator says? I want to know because I went away for vacation on June 10-17 and although I did not have sex I am thinking my ex is going to question praternity. Thanks

Answer
Karen,


The accuracy depends upon how the due date was calculated - using the first day of your last menstrual period isn't accurate unless you have 28 day cycles and ovulate on day 14.


But, if you had an early ultrasound, which it sounds like you did because late ovulation is suspected, then the dates are accurate within +/- 3 days, meaning you definitely didn't conceive while on vacation.


I'm actually due the same day as you, meaning we most likely both conceived around June 30th-July 2nd.



Best,
Catherine