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if you feel for the hard part of your belly, pressing pretty firm you should be able to feel where your uteris curves at the top. Most OBs will measure from your pelvic bone to that curve and how ever many inches it measures is can give you a good idea of how far along you are. But I would suggest making sure you see a dr soon if you haven't already
If you get a print out of your ultra sound it says how many weeks the baby is by measuring its head, torso, and femur. ask for that and you will know, or look in you medical chart the next time you see your ob.
To figure out how many weeks you are, you have to know when the first day of your last period was. Once you figure that out, simply count the weeks from then until today. That's how many weeks you are.