How accurate is an ultrasound in determining the age of fetus?

My boyfriend were in and out of a relationship and had sex throughout october '09 but around the end of october he left town.  At the 27th or 28th of October `09 I had a one night stand with someone. I am 29 weeks along today ( April 26th '10.) I had a 14th week ultrasound from my midwife and a 20 week ultra sound with an ultrasound specialist and they both checked out with eachother, making the conception date at October 6th and perhaps a few days earlier because of the time it take for the sperm to reach the egg. My concern is, might the ultra sounds be off by like 3-4 weeks and therefore the baby belong to the guy who I had a stand with? I'm in love with the idea of it being my boyfriend's ( were back together) baby and would be devistated if it wasnt...am i over reacting? the odds are in my favour right? any insight would be great-thanks

[xx_Louise_xx]

Hey, i guess your guilt has kicked in! But in my opinion, your safe and i would say your bf is the dad! Being off by 3-4weeks on a scan, i very much doubt that. I say dont worry, if it as me, due to your due date, i would say 99% sure the baby is your boyfriends! Hope i helped.

[k10road]

I agree with Louise.  Sometimes ultrasounds can be a little bit off, but not almost a month.  But make note of one thing - if you are 29 weeks today (based on 40 weeks), then conception occurred around October 19, not the 6th.  The 6th would have been the approximate date of your last period.  If, however, you are calculating based on a 38 week gestation, then the conception would have occurred around the 6th.  I know it is very confusing, but technically, when conception occurs, you are already considered to be 2 weeks pregnant - weird.  Either way, I still think it would be your boyfriend's.  But the only way to be positive is a DNA test.  

[gracee10]

hello! dont worry, 3-4 weeks is  alot, there is no way the ultrasound could be off by so much. ur boyfriend is the dad! and no, ur not over reacting, that is a big deal... but relax, does ur boyfriend know about the 1nite stand? if he does and u want to be super positive that hes a dad, u could alawys do a dna test but i dont think thats necessay bexcause if u went to the specialist, then its sure that ur boyfriend is the dad. congrats!  and goodluck.....

[LosingMyMindInGA]

ultrasounds in the first trimester are very accurate, especially if your 20 week ultrasound corresponded with it.