The hormone relaxin causes the ligaments in your pelvis to widen and become flexible so that birth will be easier. You may notice your hips widen, and they may also feel a little floppy from the relaxin. Because relaxin increases flexibility, it is easy to injure yourself during exercise so be careful not to overstretch your joints.
This is one of the reasons why pregnant women are told to avoid heavy lifting during pregnancy. Now, a sensible person might extrapolate and avoid certain other activities very late in pregnancy. A sensible person might figure that with 6+ pounds of fetus hovering just over her pelvis, it might not be such a bright idea to dance a quickstep.
You're probably not aware on a day-to-day basis that your pelvis is actually two separate halves that are joined in the front by a bit of ligament. You probably don't think of that bit as a joint, after all, normally it doesn't move. Of course, if you were 37.5 weeks pregnant and dancing the quickstep, that joint might suddenly pop, much like other joints sometimes pop, but with much more stretching of muscles that normally don't stretch so much...
I am not a sensible person, it would seem. I think I pulled my groin. In the immortal words of Homer Simpson, "D'oh!"