Why can't i go on rollercoasters when i'm pregnant?

Start-Stop: Rollercoasters often engage in high speed and frequent start-stop maneuvers in order to elicit the adrenaline rush. Unfortunately, these maneuvers place your changing body, the baby, and your placenta/uterus at risk (e.g. placental abruption). Furthermore, the security restraints often do not fit pregnant women correctly, and can cause damage just as an improperly placed seatbelt can.