G was adamantly against doing an amnio to check for T18, but when I called my OB back and told her how worried we were, she called the perinatologist and was able to schedule us for a Level II ultrasound this week. The peri took a very thorough look at the baby, and he has none of physical abnormalities associated with T18. The peri said he would be "shocked" if the baby turned out to have a chromosomal defect, and feels that the normal ultrasound reduces our risk level enough that he wouldn't advise an amnio. Needless to say, I am VERY reassured, and have managed to dial the worrying back down to my normal level of paranoia.
Sorry for not updating sooner, but between insane work levels and my brother's graduation from med school, crazy isn't even the word. I'm so glad the baby stress has been more-or-less removed, because there's enough other stress in my life to go around, and then some.