Expert: Catherine Beier MS CBE - 10/20/2009

Question
You did an awesome job on answering my question but I'm still alittle scared because I heard that ultrasounds can be off by more than a week and also with my cycles being longer than normal women I have been having a lot of sleepless nights. My cycles have been really reg this year coming every 35-37 days, - for the times in Jan and july, they were both over 40 days. My last known period was 6/29/09, given my cycle history I would have came on 8/3/09 and I think I did but don't remember and if it didn't come that week, I know it did come the early part of aug because i do remember that it was a very short cycle with brownish blood. Anyway, I had sex on 8/6/09 with my now ex bf (I was off my cycle by then if I remember correctly) and my ex husband are trying to work things out, we have been back together since aug 13th, we had sex on the 14th and beyond. I know sperm can live inside you for 5 days, so is there a chance it could be the first guys. My early ultrasound showed me at 9 weeks 4 days (10/8/09) with a edd of may 10th but I know they can be off too. I have gone over and over this is my head and I just can't see it being the ex bf because I don't think I ovluate early or super late, depending on how you look at it. Thank you for taking the time out to answer me once again, you're wonderful and I love this site!

Answer
Eve,

Like I said before, early transvaginal ultrasound is the *most accurate* way to date a pregnancy - it cannot be off by a week, it's +/- 3 days at worst.  In most cases, it's accurate within +/- 1 day.


Late ultrasound after the first 12 weeks can be off by a week or more.  They become less accurate the farther you go into pregnancy.



Early ultrasounds are very accurate because all babies develop at very, very similar rates during the first 12 weeks - it's only after this that factors like genetics, diet, exercise, lifestyle, etc begin to cause individual growth rates.


A 10 week fetus looks much different than an 8 week, 9 week, 12 week, etc.



So, my last answer still stands.  You haven't provided any additional information that would change things.  If your periods are irregular, then those dates would be irrelevant compared to the early ultrasound dating.




Best,
Catherine