"Currently, there are no studies that document a problem with roller coasters while you are pregnant. Similarly, there are no studies documenting that it is safe either."
http://www.americanpregnancy.org/pregnancyhealth/rollercoasters.html
"Although some pregnant women may ride them without consequences, in later pregnancy there is a risk that the fast stops and starts could lead to placental abruption. No one has formally documented the exact risk of this occurring, but the best course of action is to avoid any risk of placental abruption from a roller coaster by simply not riding the roller coaster while pregnant.
That said, riding a roller coaster in very early pregnancy (such as the early part of the first trimester) is probably not likely to cause a miscarriage. In early pregnancy, the placenta is at much earlier stages of development and is less likely to be affected by jarring movements. So if you are wondering whether riding a roller coaster before you knew you were pregnant may have caused your first trimester miscarriage, it probably didn't. "
http://miscarriage.about.com/od/lifestylefactors/f/rollercoaster.htm
Hope these help you make an informed decision!