Z.A.
As a child growing up in a medical family (I have a large family, we have several doctors and you can't swing a cat without hitting a nurse or med-field scientist)... TRUST ME. no one, but no one gets more "2nd" (aka 3rd, 9th, 20th) opinions than doctors themselves. Oy. They know all the things it COULD be, you see... and worse know half the personalities in the field... so it goes from dx'ing themselves, to having their colleague in the office take a look, to the specialist they know from 'x', to the top guy/gal in the state... all the way to Johns Hopkins/ Mayo, you name it... when they're either not satisfied with the answer, or have decided to seek out the "best". It can get downright amusing to be watching this from the sidelines.
MANY doctors can & do get affronted and snippy when their patients 2nd guess them. (Even when they've just seen 4 people last week about "y"... my grandfather used to joke he took 2 oaths, the hippocratic and hypocritic). <grinning> So too darn bad. At the end of the day you go home to your family and they go home to theirs. Take care of you and your own, and they can take care of theirs. So NEVER worry about offending a doc. If they get proved right, the get to gloat in secret and if they get proved wrong it's interesting. And anyone who isn't confidant enough in their own dx to have someone else *good* to look-see, you don't want to be seeing them anyway.
For a 2nd opinion I would highly suggest getting not a 2nd OB opinion, but an Internist.
(As a NON-doctor) it strikes me that while the symptoms triggered with both pregnancies, and have become chronic with the 2nd... it seems a stretch to assume that the problem is part of the reproductive tract. It COULD be, of course (endometriosis, a cyst, a nerve grown the wrong way)... but everything gets shoved around while we're preggo, organs/nerves/major blood vessels get rather rudely elbowed out of the way. Add in massive hormones (which can trigger all kinds of things), and "referred pain" and you've got yourself a puzzle. Now... while there are general internists, most specialize these days into subsets of gasteroenterology/ cardiology/ neurology/ endocrinology/ etc. (I think there are 13 subspecialties, if I'm remembering correctly) What I would recommend doing is to call up your favorite hospital, ask for their doctor referral line (or get one from your GP) for an internist to help dx severe abdominal pain.