Is it safe to go on a roller coaster while pregnant?

Don't Do It!! The decelerating forces and sudden drops are enough to cause ligament trauma, pelvic pressure/pain, & worst of all - placental separation which is very dangerous (abruption). It's best to avoid roller-coasters during your entire pregnancy.
Only very early. Roller coasters have lap bars that go against your abdomen. Once you're past 16 weeks of pregnancy, the uterus is in the abdomen and can be damaged by jostling against the bar. During the earlier weeks of pregnancy, the uterus is in the pelvis and well-protected, but combining morning sickness with roller coasters sounds really miserable.