The same thing happened to me. They noted a low lying placenta at my 18 week u/s and recommended a follow up at week 29 at which point they told me my cervix was shortening (I don't remember what the measurement was at that point). They sent me to the hospital to monitor for contractions (I wasn't) and gave me steroid shots in case I went into pre term labor and basically scared the pants off me. The OB on staff at the hospital (my OB was on vacation) put me on modified bed rest and I started going for cervix checks weekly. I spent the first week taking it easy but not really doing bedrest and at my next check my cervix had continued to shorten. When I saw my OB she basically said what RCW said, that bedrest is useless but that I should continue to take it easy and that she figured I would have the baby mid December (I was due Jan 6).
At 36 weeks they told me I was 2 cm dilated and would go any day. Then I was 3 cm. Then 4. And NOTHING, not a single contraction, NOTHING. I walked around 4 cm dilated for almost two weeks. Every week my OB would be all oh, make an appt for next week but I won't see you here, har har har. All through this they kept asking if I was having contractions and I wasn't.
I ended up going 41 weeks and when I went into labor I went from my very first contraction to having them 4 minutes apart in 1/2 an hour and when I got to the hospital I was 7 cm dilated.