Ultrasound is notoriously innaccurate and gets more so as time goes on (it is something like +/- 7 days at 8-9 weeks but +/-3 weeks as you approach full term). That said, very early ultrasound (before 8 weeks) is supposed to be extreamly accurate. Definatley within a few days. If I understand correctly, you are asking if a 6week6day ultrasound could be off by around 6 weeks and, as far as I know, it just couldn't be.
Here is a fetal development chart:
At six weeks the embryo is 1/2". At 12 weeks the fetus is around 3".
Here is another fetal development chart starting with conception rather than LMP:
So, at 6weeks6days you would be approximately 28 days past conception. Using October 29th instead of December 10th as date of conception would add another 42 days (70 days). That wouldn't even be shown on the chart above! You wouldn't be looking at a blob in a sack--- you'd be looking at limbs!
This picture is 3-4 weeks from conception:
Here is 12 weeks (unsure if from LMP or conception, but the difference is HUGE):
When did you test in December? Was it before 12/10 or was it late in the month? Sometimes stress can cause you to ovulate late which could explain why you didn't have a period in November. Did you have one in late October? You said you normally get it around the 26th which would mean you were still bleeding while with the October partner making it very unlikely you concieved at that time.
Yes, ultrasound is horribly inaccurate. I have so, so many stories from family and friends on the unreliability of ultrasound. I have known women who were told that their babies had passed only to request one last ultrasound "just to make sure" just before a scheduled D&C and found a perfectly alive and happy baby (whew!). My coworker had a boy early due to severe hyperemesis and the doctor said it was okay to just take baby out because he was 5 lbs. Turns out, baby was only 2 lbs and survived by what miracle, I have no idea. I'm glad everyone turned out okay, though! My SIL's second-to-last baby was born by cesarean necessitated by induced labor because the doctor said that the baby was too big - over 10 lbs - and had to come out early. She ended up being only 8 lbs and boy, was my SIL ever peeved.
I really, really doubt it.
Although ultrasound is notoriously inaccurate as far as some things go (weight estimates in the third tri comes to mind), the odds of being off by 6 weeks at a 7 week ultrasound have to be, well, zero. There is a big difference between a 7 week old embryo and 13 week old fetus. HUGE. Likewise, if you were six weeks further along than originally thought, you'd be approaching your due date in a couple weeks. How are your fundal measurements? Are you measuring 38-ish, or are you measuring 32-ish?
On my part, ultrasound had been accurate,..in terms of gender and due date. But I've heard some people are given inaccurate gender.. Is it the Ultrasound machine or the professionals reading the results?? But ultrasound before giving birth is really essential. My baby had a cord coil(3x), we have not known it until she was born.I'm still lucky that even though I have not done a Ultrasound before giving birth my baby survive!.Thanks God!!
Living is great but being a MOM is even better
Holly and David
Adaline (3/20/10), and Charlie (1/26/12- 4/10/12) and our identical twins Callie and Wendy (01/04/13)