Wear trousers or a skirt not a dress!
Basically make sure you drink loads before as this gives the sonographer a far clearer pic. Sit with your legs crossed in the waiting room till they call you in.
You will lay down on a bed and they will pull your trousers down so they are right at the top of your pubes, and squeeze some gel onto your tum (this is normally bloody cold!) Then use the doppler thing to have a look. They normally press very firmly with can be a little uncomfortale (esp with a full bladder lol) But its not painfull at all. There is no sound so dont expect to hear a heartbeat. Sometimes it also takes a little while for them to find a ood angle to see your bub, so dont get alarmed if its a couple of minutes before they speak to you. Normally they will turn the monitor to face you so you can see it on screen and they will point different bits out to you. Sometimes the little bbean is ouncing around like mad, other times they are asleep so wont move at all, so dont be worry if they seem very still. The sonorapher will use a mouse to draw lines on the screen taking measurements so they can get an acurate date on the baby, which may be a few days either side of what you calculated previously. They will take a few still shots of the bean and you can normally then choose which ones you want copies of. A lot of hospitals charge for these pics so take a little cash with you. (normally between £1-£3 each). They then give you some tissure to wipe off the gel, and your free to go to the loo, while they just write up in your notes their findings And you just wait till they give you back your maternety notes. They normally allow one other person in the room with you to see, and if you have children they can come in too so long as they are well behaved.
Its all a pain free procedure, and dont worry its normal to be ricking it before hand, worrying if everything is okay!
