I have a fibroid that was only discovered during my first pregnancy because I could feel this strange, painless, large mass that moved about in my abdomen. I asked the sonographer to have a look at it when I had my nuchal fold scan, and he identified it as a fibroid, external to the uterus, and at that time about 8cm diameter. It eventually grew to 11cm by about 35w (my last scan - I had many, but nothing to do with the fibroid). It gave me no problems whatsoever until about 20w, when it must have twisted on its stalk, because I ended up being monitored on the labour ward as the pains were so excruciating. It settled down again within a month, just giving me occasional twinges. I don't think it ever 'died' because it kept growing, but after the middle of the pg it only gave me positional pain - eg bending or straightening my legs. The fibroid had absolutely no influence on my labour. It shrank a little afterwards, back to about 8cm, and (probably) caused me very heavy periods. It also caused me occasional positional twinges, but not nearly as bad as during pregnancy, more like trapped wind.
Because the fibroid is external it moves fairly freely around my abdomen, but I have only ever noticed it moving during the first pregnancy. With the second pregnancy it was much lower down in my abdomen - I didn't feel it move at all this time. But it gave me excruciating pain (gasp-and-fall-over pain) until about 12w, and then seemed to disappear. Because the fibroid was so low, there was concern that it might affect the labour and I might have to have a caesarian, so I was scanned again at 36w. But by then it was well above the level of the baby's head, so again there were no problems with labour.
Since the birth I haven't any problems from the fibroid, and my periods are lighter than they were before having babies. Perhaps the fibroid died, because it measured 5cm at my last scan.
Redshoes, with no disrespect to the experts, I don't think an ordinary sonographer is the person to rely upon to decide on all the issues relating to your fibroid. If at your next scan the sonographer thinks there is a problem, then you should be refered to a gyne, or at least seen by a senior sonographer, to decide whether the fibroid is a problem.