Best way to feel your fundus (post orgasm it is easier--thats how I found mine also jess! hehehe)....
lay flat on your back, RELAX your stomach muscles, depends on how far along you are but a good starter point is your belly button. Slowly push down (towards your spine) if you don't feel a hard lump that feels like it moves your entire lower abd keeping moving your fingers closer to your pelvic bone till you do. Mine is pretty deep still so I can only really feel it post Orgasm ...because its harder then AND i'm nice and relaxed

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At 20 weeks you should be at belly button height. for every week Less then 20 weeks you are you should be one finger width (1cm) lower then the belly button, for ever week you are more then 20 weeks you should be one finger width higher then your belly button. This is a rule of thumb from 15-36 weeks along. If you have a tilted uterus it will take longer to feel it because it lays back so far, and the more stuff you have to push through the harder it is to find. OH pee first!! LOL if your bladder is full you will most likely feel that (and your uterus will be tipped to the left). It takes some practice to find out if its fundus or poop though LOL...it gets better with practice.
OH and FYI---we like your fundus to be a least back down to belly button before we let you leave the hospital post birth.