I am measuring small too, and it can be very normal and quite common. In early pregnancy I was measuring large. Babies are all different and have different rates of growth. In fact, I am pretty sure next appt I will have caught up because I feel like I have less room to hunch over and breathe these past couple of days. I too have only gained 8lbs so far, but that is because I was overweight to start with and eat better now.
I would never worry about a client unless their fluid felt off and the baby did not feel like it grew or measured like the babe was growing over a period of 2-3 appointments...then I would go for an u/s. And by feel I mean when palpating a baby, which should be done at each appointment. Also, you can be measuring low or high for dates but as long as there is growth that is fine too. For example, 30 weeks measuring 27 then at the 32 week apt measuring 29...etc...
Once when I was a student we had a woman come in for an initial appointment who did not really have a good idea of her dates. She measured something like 32 weeks to me and my supervising midwife...she was also quite thin and hid the baby well... Anyway, we consulted her for an u/s to get an idea of her dates but she returned in heavy labor a couple of days later, before she had gotten the u/s. You can imagine we were quite distressed that she would give birth before the ambulance could get her to the hospital and we would have a preemie resus to deal with. But the ambulance came and took off with her and a student. The student came back a couple of hours later to tell us that she had the baby before they could even make it up to L&D and it was a 7 pounder! That really taught me a lesson about how valuable measurements can be and that the most important thing is consistency in growth over a period of time, not the actual one time measurement itself ...because, like someone else said, all of our bodies are different.
If you have different practitioners doing the palpation then they may be more likely to want you to have an ultrasound because measurements are subjective and can vary so much between people. They could also just be a conservative practice and do more high tech things more easily.
As far as u/s go, I would take any later in pregnancy, esp 3 trimester u/s measurements and weight with several grains of salt. The only truly accurate ones are in the first trimester...by the third dates can be off by a couple of weeks and pounds can be off by a couple as well...and then you have to also take into account the skill of the person...but, no matter how good they are they are statistically inaccurate...just a guess for weight and dates by the third trimester. This is not to discount that other anomalies can be caught which might cause a discrepancy in growth at an u/s but that u/s is not a perfect science at all and some things are not caught too. A high level u/s is a clearer picture, so if there is real concern I would make sure it is at least a level 2.
And, I have seen folks on the McDonalds diet have very healthy babies so if you are eating well this is a bonus for your baby, but even more for your body in the long run and your family as an example of good eating to them...so I hope that also makes you feel better.
Yes, there can be congenital issues that cause growth or fluid issues but they are rare and from what you have described I would not jump to conclusions at all about what is going on. Perhaps you could call the office and ask for someone to talk to you about your measurements over time and your fears and that might make you feel better. Good luck and let us know how it goes. Paige