If you're enjoying a normal, healthy pregnancy, it's a good idea to carry on doing some exercise, providing that you're not undertaking a strenuous new regime. However, you should check with your GP, or midwife, that exercising won't pose a risk for your specific medical situation.
Cycling is a good activity to do while pregnant, but it isn't usually recommended after the second trimester of pregnancy (after 26 weeks) due to the risk of falls. However, you can use an exercise bike for as long as you like.