Size 18???

So I went to David's Bridal yesterday to try on a bridesmaid's dress for my sister-in-law's wedding in May.  At that point, I'll just be exactly 6 months pregnant.  Unfortunately, I have no idea what size I'm going to be.  Here is what the dress looks like.

Now, one would think that that would be an excellent choice for someone trying to smuggle a large basketball (or maybe larger?) into a wedding.  Well, the problem is that under the loose chiffon is a fitted satin layer.  The chiffon, of course, is translucent, and then it has a fitted lining.

A size 10 fits me like a glove right now (3 months pregnant).  The 22-year-old stick-thin women at David's Bridal recommended that I get a size 18 and have the dress completely reconstructed (which the alterations guy said would be "very, very expensive").  Does anyone out there know anything about this?  I have no idea what size to get or how to get this altered to fit me.  I don't want to get it too small, but an 18 sounds insane to me.  I wonder if I could get a 14 and try to find extra fabric at a fabric store that closely matches the liner so that I could have panels added to the abdominal area of the lining.  I'm at a loss of what to do here.  Any seamstresses out there?