Sayang, the bumpy ride itself won't directly cause miscarriage, tapi early pregnancy memang sensitive and prolonged jolting can make you feel uncomfortable and crampy lah. Better to check with your doctor first, especially if you're having any bleeding or cramping already, they know your pregnancy history best and can give you peace of mind. A few of my mum friends avoided long lorry rides dalam first trimester just to be safe, even though most pregnancies are quite strong. Rest as much as you can after the journey okay, and if you notice any spotting or severe cramping afterwards, go see your gynae right away.