Thanks, Lisa, for all of your encouraging comments and bringing up Braxton Hicks contractions.
Yes, it seems these are Braxton Hicks contractions I am experiencing during exercise. These are also referred to as "false labor" but interestingly begin as early as 6 weeks into the pregnancy and are not perceived by the mother until about the 28th week. Exercising is known to increase the frequency and intensity. I can't find a good definition of them and I think that is because it is still not understood what their purpose is or how they are related to "true labor".
So I did a bit of research on them in the medical literature. All I found was Braxton Hicks contractions have never been shown to have an adverse physiologic affect on fetuses in normal pregnancies. In fact, they seem to increase heart rate variability and breathing variability, which can only be a neutral or good thing.
It was also believed by John Braxton Hicks who described them in 1871 that they improve blood flow to the fetus and "adapt... the position of the foetus to the form of the uterus." (what that last part means, I am not sure). Here's JBH:
It is further theorized today that these contractions soften the cervix, increase the strength of the uterus and thus the more you have, the easier and less painful your labor will be. None of the last part is supported or refuted by any data. But, what the heck? Anything that might make labor a little easier and has no adverse effect on the baby is worth a shot.
Is Braxton Hicks not the perfect name for a saxophone player? I'm sorry, I just thought it was so funny that this jazz man shares a name with false labor.