I find it impossible to believe that I'm supposed to keep my heart rate under 150 bpm.
Your instincts are correct on this one. I have been reading all about exercise while pregnant and it now proved to be the "old way" or myth that you can't bring your heart rate above 150bpm. The main concern is temperature. The only evidence they have found that intense exercise can be harmful in any way while pregnant is when the mothers temperature rises above a certain point (I can't remember what it is).
I have read many stories of women who continue to run throughout their entire pregnancy even all the way up to the day they gave birth. One girl said she monitored her heart rate which usually went between 165 and 170bpm while she was running. Her and the baby were fine and she gave birth to a very healthy baby. But then again this girl was a very experienced and conditioned runner.
I do think the rule of thumb of not starting any new exercise while pregnant is good because of the hormones and relaxin your muscles may not be able to take new strain on it. However, If you are already a runner or person who does certain exercise I think it is perfectly fine to continue throughout the entire pregnancy. But then again, I am not a doctor, this is just from research online and other women's stories.