HR= heart rate.
Most doctors will tell you to walk or swim for cardio when pregnant. Some people ride right up to their delivery, but others stop in the last trimester, because of balance issues. Riding on a trainer or spin class would be a good alternative. My "babies" are now almost 24 and 26, so my experience is old. I did high impact aerobics through both pregnancies, though probably I did too much! I modified a lot of stuff near the end. I had to be on a sort of modified bed rest for the last 6 weeks of my second pregnancy because I went into premature labor, probably as a result of a bike accident, where I stopped short and the down tube shifters on my 80's Univega went directly into my bulging belly. I most definitely was not listening to my doctor there.
The HR thing that some say is not to let it get above 140 and do not lie on your back, like for sit ups, as it compresses the major artery that supplies blood, and hence oxygen to you and the baby. There is a lot of conflicting support for this and some say that it depends on your condition, i.e. if you are highly trained then getting your HR up high will not shunt as much oxygen away from the baby's needs.
I think I gained 6 pounds in my first trimester with both pregnancies. I was very thin (like less than 100 pounds) at the start of the second one, but I gained at about the same rate for both, until they put me on meds to stop the labor, which actually caused me to lose weight. I had to significantly "eat for two" during those six weeks, but in a healthy way.
All the exercise paid off after the babies were born. I went home from the hospital in regular clothes, a couple of sizes bigger than my usual size. I lost the remainder of the weight in 3-4 months, with continued exercise 5-6 days a week. I didn't have to starve myself to do this, just eat healthy foods.