It is probably nothing to worry about, but since you're asking for ideas, I thought I'd throw this one out...if you notice any digestive issues (excessive diarrhea, vomiting, not eating regularly), you might have that checked out.
My daughter never had a height/weight issue, but has had digestive issues from the time she was diagnosed with juvenile arthritis (age 2) . At age five, she was finally diagnosed with ulcerative colitis. Most of the kids I have seen who have ulcerative colitis or Crohn's are very underweight.
It's pretty uncommon to have a GI diagnosis at 18 months, but I heard a pediatric gastroenterologist speak about infants in his practice.
If you are concerned about it, definitely get a second opinion. It took four years of hearing "your daughter's digestive issues are nothing to worry about" and then "your daughter MUST be lactose intolerant" (she has tested negative to lactose intolerance) to finally get her ulcerative colitis diagnosis. When she was diagnosed with juvenile arthritis, I never felt like that was all to the diagnosis. I did a ton of research, asked lots of questions, networked like crazy with other parents. Then my daughter developed uveitis (inflammation of the eye) and I dug further down that path with my research.
I finally attended a conference where I got the link confirmed among the arthritis, uveitis and gastrointestinal issues. Even with the medical terms, my daughter's many medical specialists told me there was nothing else to her long list of health issues! I stuck with my motherly instinct that said something wasn't right and finally got the right tests done to show inflammation of her intestinal tract. A colonoscopy confirmed the diagnosis.
So.... if you feel like there is more to your daughter's low weight than "she's just going to be small", I highly recommend getting a second (third, fourth, etc.) opinion until you feel comfortable there is nothing to worry about. I wish I had pushed my daughter's many doctors (6 of them!) to look at her intestinal issues more closely, but I thought that all these doctors have a lot of combined medical experience and I have no medical background so they must know. If it had been just one doctor, I probably would have pushed harder, but six of them all said the same thing.
Sorry for the long, rambling response, but I'm really passionate about urging moms to listen to the concerns they have about their child/ren and keep asking questions until you get answers that make those concerns go away.
Good luck!