I love recommending spinning during pregnancy. It is a great way to get some exercise and have some cardio.
A couple of rules, though. Wear a heart rate monitor. You need to know where your heart's at. Also, talk to your healthcare provider first and set up a maximum heart rate for yourself. Then watch your heart rate monitor to make sure you don't go above and exceed that rate.
Also, you want to make sure that you hydrate before, during, and after your spin class. As you probably already know, spinning can dehydrate you quickly. When you're pregnant, it happens even faster.
Once you start having a belly, you're going to have to start to adjust the bike. You'll take the front handle bars up a little higher as the belly starts to grow so you don't have to lean over and compress your abdomen.
You may also need to adjust the front and back position of your seat, of your saddle, so that your knees aren't hitting into your belly.
Then you are going to reach a point when the belly, no matter what, is going to be too big to even adjust your bike any longer. At that point you may have to get off the bike and find another form of cardio.
When you're choosing your bike in the room, you want to aim for a bike that's near a window, door, or fan so you can get more ventilation and be less overheated. Again, drinking water will assist in avoiding overheating.
Make sure to go at your pace and at your resistance. Even if the teacher's saying go this fast at this RPM, you are welcome to back off. You don't have to put the full resistance on. It's good enough for you to just stay on the bike at a comfortable pace.
Finally, if you're in class and you start feeling dizzy, nauseous, too overheated, if you start feeling abdominal pains, or after class you notice vaginal bleeding, it may be time to get off the bike. If this continues, please see your healthcare provider or doctor.