I have a large fibroid on the outside of my uterus, and it also gave me severe pains in pregnancy. However, apart from pain, it caused me no problems whatsoever and my labours were perfectly normal.
The pain is caused, IME, by two things: Firstly, if as in my case, the fibroid is on a fairly thin stalk, it can twist around the stalk. Secondly, the pregnancy can cause the fibroid to grow quite fast. If it outstrips its blood supply it may then 'die'. This is also very painful, but has the advantage that the fibroid does not then continue to grow or cause problems.
First pg: my fibroid was 8cm diameter when detected on a scan at 11w. It was painless, but could be felt from outside, and, freakily, I vcould feel it moving around, literaly swinging around inside me. It grew to 11cm diameter and caused me severe pain at about 20w. It didn't hurt as much afterwards, but niggled strongly a lot.
It shrank to about 8 or 9 cm after the pg and caused me very heavy periods, but nothing worse. I had investigations but decided not to have it removed yet because of the risk of having to have an emergency hysterectomy.
Second pg: the fibroid gave me dreadful pains between about 12 and 20w (I don't remember exactly). Not aches lasting a long time as in the first pg, but sudden stabbings. It didn't grow at all during this pg. I had a late scan to make sure that it was above the baby's head so out of the way for labour.
No problems at all afterwards, periods if anything lighter than pre-babies.
third pg: the occasional "Don't forget me - here I am" twinge. Apart from that - nothing. At an early scan it was found to be agbout 5cm.
I think in the first pg the pain was caused by twisting, in the second by the figroid 'dieing'.
Although I wanted the fibroid removed after teh first pg, it now doesn't bother me at all, so I'm doing nothign about it.
HTH