i think the subsequent babies typically come a little earlier - the experience so far for my sisters and I. my second one was born premature - exact cause unknown. I had strong contractions from middle of 2nd tri onwards (maybe because I was still latching no. 1 on). and was losing weight from nausea. and by 3rd tri, baby's growth rate dropped dramatically and she decided to come at 35 weeks.
it could also be likely that the dating was not so accurate so maybe your baby is not as old as thought to be? hang in there! it won't be long!