I asked a similar question about a year ago, and I found this post helpful for the reasoning of the first trimester pelvic exam:
If the pap is due, we usually go ahead and do it in the first trimester, along with STD swabbing, if mom wants. We haven't ever had any problem of a mom having a miscarriage or any problems after the pap.
One thing is that we always wait until the second appointment. I think it is a terrible howdy-do....hello, I'm your midwife, welcome to the practice, now strip down and let me poke around a bit.
On our first visit, we always just go over the client's health and personal history,and draw lab work.
One thing our midwives like to do is measure the cervix. It has happened a coupel of times that a mom has a naturally short cervix. Then, if mom has any problems later on, and a VE is performed that finds a short cervix, it is noted on the chart already whether that is normal for the mom, or whether some change has occured. It has kept a couple of moms away from that "threatened preterm labor" label.
And actually, since we do well woman exams, too, we now note the length of cervix on every woman of childbearing age, so that it doesn't come into question later. I know that cervical length can change, depending upon where one is in one's cycle. But I still think it is good to note (along with where she is in her cycle), for possible future reference.
Pelvimetry (measuring the pelvic outlet, and checking what "type" of pelvis you have) is somewhat helpful. But, unless something incredibly odd is noted, it isn't extremely informative either. Until labor, no one knows how much your pelvis will relax and open up. So much depends on such variable factors---baby's position, mom's position and activity, etc.
If you don't want the exam, it isn't so important that you MUST have it. Just refuse politely. It shouldn't be a big deal. For some moms, we would rather know it is a big deal, and avoid it all together. Just tell your provider how you feel -- you aren't at risk for STDs, you are sure of your dates, you are current on your paps, and you simply don't want anyone else in there at this time--plus, your pelvis is "proven" in that you have already had a child. I would rather have one of our clients refuse the exam, knowing that she was "feeling protective of that space." If she is feeling that protective, that is a good sign, to me, that she is going to take great care of herself, for her baby.
HTH
blueviolet - you're awesome! 
