some babies are just small! My eldest has always been in the 10th percentile for weight, one checkup she was in the fifth! They really start to worry if they show signs of malnutrition or if they aren't growing "up" on schedule.
When ny daughter was in the fifth percentile they said to let her have lots of fatty, yummy, higher calorie foods. No "diet food"- while still limiting junk food like candy, cookies, salty chips etc. My pediatrician said "let her have sour cream and butter on her potatoes, or dip her veggies in thick creamy dressings... and don't worry about the fat in her foods.
She is still a bean-pole... but her height is at the 90th percentile. I was always skinny, in fact I was 16lbs at 18 months. (if you have your own baby book, compare your weights to hers at the same ages... were you healthy as a kid?) As long as they are alert, generally healthy, on track and growing- don't worry. If there is reason to be concerned your doctor will have you do more, like give her pediasure or a prescribed calorie intake.
-M.