Here's some info I found online:
"Rapid acceleration and deceleration, prolonged multi-gravitational
(g) force and high speed rides are all common thrills in the amusement
park, exerting inertial forces of mass versus momentum. These forces
exerted on the pregnant rider may result in rupture of membranes,
abruptio placenta with immediate or delayed symptom presentation, vena
caval occlusion and fetal maternal haemorrhage (Buchsbaum, 1968;
Crosby et al, 1971; Crosby et al, 1968, Crosby, 1974; Mengert et al,
1953).
"Extensive studies of motor vehicle accidents during pregnancy have
found that violent motion of the uterus may cause injury, even in the
absence of an abdominal blow or any soft tissue injury to the mother
(Crosby and Costiloe, 1971; Crosby, et al 1968; Crosby 1974;
Pritchard, 1970). Animal studies have shown that during rapid
deceleration, the pregnant rider's body flexes forward and abdominal
organ compression elevates the intrauterine pressure and distorts the
uterine size and shape (Crosby and Costiloe 1971). In one report the
maximum pressure recorded was ten times that observed in labour
(Crosby, Synder, Snow et al 1968)."
I would think at such a precarious time in your pregancy that you wouldn't want to take any chances. Personally, I wouldn't ride any of the rides that have the advisory. Yes, most of it is a CYA thing for Disney but it is based on some science.