During the first trimester, your pregnancy is protected by your bony pelvis.
So, if you fall off the bicycle, it is unlikely to cause harm to the fetus. However, after 12 weeks gestation, the uterus is larger and enters the abdominal cavity. A fall off the bicycle could then harm the fetus. So think about switching to the stationary bike instead. You’ll still get a good workout without having to worry about a fall.