Kind mamas, thank you for your encouraging and helpful responses!
DD2's feeding problems were related to a structural problem with her esophagus (as it turns out, it was squeezed almost shut in the middle by an extra arm of her aorta -- which is now removed). She also had trouble swallowing and breathing simultaneously due to huge adenoids, which were removed in March. That's when she started eating really well and heartily, and when she started putting on weight. The remaining food issues are mild reflux treated by prescription antacids, and esophageal dysmotility that resulted from her esophagus being squeezed while it was growing and developing (basically, sometimes it doesn't move food down in a coordinated way, and DD2 will suddenly sit up very straight while eating, crane her neck sideways, and ask for a drink). None of these things seem to have much effect on nutrient absorption that I know of, but I'm curious if anyone knows differently or has varying mileage on either issue.
We eat a pretty balanced vegetarian diet around here. She and her sister eat dairy and eggs -- sometimes not every day, but certainly every other day -- and lots of fruit and veggies and good fats like olive oil and seeds and beans, whole grains, etc. I've never given DD2 a vitamin, but plan on starting this winter. I would call her diet balanced and healthy, but her pediatrician gave me some gentle suggestions that vegetarians are known to be about an inch shorter on average. I said I wasn't worried about one inch in the span of her whole lifetime -- that's not worth worrying about for me. I don't really think adding meat is the solution.
As for the rest of us, I am pretty short -- about 5'1" -- and so is my mom. DH is absolutely average for a guy -- 5'9" -- and DD1 is smack dab in the 50th percentile. However, even my bitty little 5'1" is still around the 10th percentile, not 3rd percentile or littler like DD2.
I think we've decided to wait another 6 months and see how she does. If she grows an inch or more in the next 6 months, we'll forget about it. If she doesn't grow at least that much, we'll look into the thyroid panels and hormone levels, etc. I hate to get started with yet ANOTHER specialist whose job it is to find problems, you know?
Thanks, everyone -- and if anything jumps out at you, please do share. We missed her extra aortic arch for a year because doctors kept saying her health problems were colic or floppy larynx tissue or bad colds. I don't want to miss something else.
ETA: Developmentally, DD2 is totally on track. She's active and energetic and smart, and has always met all the milestones. The only one she hasn't met is "pedals her own tricycle" -- because she can't reach the pedals!