You can't prevent them, b/c it's your body's way to prepair for the real thing. Often you won't feel them at all, but usually it will feel like your abdomen went from very soft to rock hard. It will affect your whole abdomen, all at once. It doesn't hurt, but it does feel odd, heavy. After initial couple of weeks, you don't even notice them unless you happen to touch your stomach only to realize it's rock hard. Hope this helps.
Actually, BH are a good thing. Like kiedy said, your uterus is prepping iteself to actually push out the baby.
When mine started, it would come on really slow...like 20 seconds or so to tighten...then it would hold tight for about 15 seconds...then another 20 seconds or so to go back to normal. I pretty much knew exactly what it was the first time it happenned...that was around Week 25.
As the weeks went on, they occurred quicker...taking about 5 seconds to tighten.
Now at 30 weeks, many times I don't realize I'm having one if it comes on while I'm sitting down...but I sure notice how tight it is when I'm in the process of standing up. I have at least 1, sometimes 2 a day. They should not hurt or be painful...just strange & tight. My doc says if they truly hurt, I'm to get myself to the hospital.
Yes I agree w/ Gayle, the first time I started getting them around 28wks, they were very noticable to me, often when I would get up, I had to hold my belly b/c it felt like it would drop to the ground. But now at 36wks, they are much more frequent and less noticable. I think I just have gotten more used to them now. Walking is supposed to make it go away faster, where as w/ 'real' contraction, walkingn will not make it go away.
Alternatively you might never notice them. I'm nearly 38 weeks and I'm not sure whether I've ever felt a BH. I've read that everyone gets them but some women just don't feel them. I'm hoping that if I haven't felt the BH's the real thing might not be so bad....wishful thinking huh?