Not as far along as ob gyn and ultrasound measure

I had my first ultrasound today via transvaginal.  It showed that I am 10 weeks along, but I know for a fact that I can't be any further than 8 weeks. It simply isn't possible, my husband is in the military and was on deployment and from him returning the earliest I could have conceived would have me 8 weeks and 4 days along.  I tried to explain this to my ob gyn but she just stays firm in telling me that I am further along than I  thought.  As I've understood, ultrasounds are pretty accurate.  Should I be concerned? Why would the ultrasound be showing I'm further along than possible?

[AnnieBrooke]

Well, you might actually be in agreement.  If the following is too basic, I apologize.  But are you clear on the difference between the length of a pregnancy as a doctor counts it, versus the length of a pregnancy if dated from the moment of conception?  If not:

The medical way of counting pregnancy --
First day of period is Day 1.  (Bleeding stops around Day 4, 5, or 6.)  Conception is assumed to be approximately Day 14, 15 or 16.   10 weeks pregnant is 10 weeks from the first day of your last period.  It would be around 8 weeks from conception.

The way most women think of pregnancy --
Somewhere around two weeks after my period came, I ovulated.  I got pregnant that day or the day after.  My pregnancy count begins the day of conception.

The medical way of dating pregnancy gives you a 40-week pregnancy, when in fact pregnancy from conception to birth is 266 days long.  The extra two weeks are to take you back to the first day of your last period.

This day and age of early detection of pregnancy, the count that begins on Day 1 of your period seems hard to understand.  But back in my mother's day and her mother's day, and when doctors began to apply science to measuring pregnancy, the ONLY thing they had to go on for a start point for determining norms of growth at different stages was the first day of the last period the woman had before she got pregnant.  There were no early tests and there was no ultrasound.  So, all the medical books calibrated accordingly, all medical schools taught those norms, and all doctors, ultrasound techs, nurses and labs use that language.

It really sounds like the doctor and you were saying the same thing, with you thinking "from conception" and her thinking "medical length of pregnancy."  I don't know why doctors are so unaware of the confusion this causes (and worse, if someone has a mistrustful husband).  Often, someone will say "How far 'along' am I?" and the doctor will answer medically, without thinking that perhaps the person was asking when they conceived.

Again, if you know all of that, I apologize .