If you aren't sure when you ovulated, which is pretty easy if you're cycles are 28 days and you know when the first date of your last menstrual period was, that's the most accurate way.
If you didn't have your period and weren't measuring your ovulation or your periods were erratic, then it's more difficult to tell, unless you know when you had intercourse and there weren't any other incidents
There is something to be said by how large your uterus is by feel, but it isn't the best indicator, esp early on and w/a 2nd baby.
sonograms are most accurate early on, before 18 weeks? After that, they aren't as accurate because that's when babies start to grow at different rates.
After about 22 weeks the length of a woman's uterus, from public bone to the top of the fundus, is indicative of how far along her pregnancy is - it's pretty amazing, actually, for most women your uterus actually measures at the same week as your pregnancy. - ie: 27" = 27 weeks.
If you thnk you're farther along than they do, insist that they keep that in mind regardless of their measurements because you don't want to have to deal with the pre-mature situation when you get there. You know your body better than they do.
You might try asking this on the midwife and doula forum, too.
Hope it works out for you.