Watch "I Didn't Know I Was Pregnant" on TLC, and you'll feel much better about all of it! Healthy babies born to women who had no clue, gained no weight, and did no prenatal anything. LOL!
Seriously, I understand that you have concerns, but as long as your doctor isn't concerned about your weight gain, you're probably fine. Some women gain very little -even if they were small to begin with -and they have very healthy babies. Also, regarding fundal height, every woman carries differently -even from pregnancy to pregnancy! I wouldn't worry about it if it's only slightly less than normal. Continue to eat healthy, take your preanatal vitamins and maybe throw in some extra peanut butter or cheese. You'll probably gain at least 10 more lbs (you'll be surprised how it can pack on "magically" in the 3rd trimester) by the end of your pregnancy.
Most likely you being premature will have nothing to do with your baby. It really depends on why you were premature.