Hi B.,
I have many years of midwifery training, though by the time I was ready to start getting ready for my actual license, my third baby was born and I decided to raise my own babies before going any further.
It is very uncommon for a FIRST baby to drop prior to early labor, but it is very COMMON for a second baby or later one to drop many weeks before the estimated due date. This is mainly considered to be because the body already "knows" what to do, so it loosens things up a little earlier. With my second baby I felt like I was walking around with a basketball between my legs for about a month and a half!
Not to worry, it doesn't mean you are threatening pre-term labor. The signs of that would be any unusual cramping (a bit of ligament pain as everything stretches at this stage is very common, though, so don't panic if you have twinges, even if they hurt quite a bit. Cramping is more overall and continuous), bleeding/spotting, losing your mucous plug, that sort of thing.
Don't let anyone scare you--not even your doctor--into any kind of interventions unless they can show you medical evidence of a problem. But this is pretty normal.
Blessings on your birth!