Hi, I got a 3rd degree tear with my first child and she was only 7lbs 4oz..... I didn't actually feel the tear happening, as everything else hurts so I just didn't notice it. My tear was apparently quiet bad, but I had no pain or discomfort whatsoever with it afterwards...... I guess the Dr who stitched me up did a good job. It did take a while to heal properly, and the idea of sex after was a bit frightening... but all was well! smile
I am 19 weeks pregnant with #2, and my midwife has suggested I do more pelvic floor exercises, as well toned muscles stretch better, and just explained that with this next birth, it needs to be controlled really well (when she say's don't push.... DON'T PUSH!!!)
Honestly, don't stress about it.... if your going to tear your going to tear, and theres not much you can do about it.... on the other hand, your bub may just sllip right out!! (fingers crossed).... Good luck.