It's not unusual for growth rate to slow down during toddlerhood, especially if your daughter was large at birth, but you and her father are on the smaller side. Essentially, her body could be slowing down to a rate that will allow her to reach her normal expected adult height at the right time.
A true growth disorder is defined by not only the percentile, but the growth rate. Children will usually either be in the 1st to 3rd percentile for height (not necessarily weight), and will have a growth rate (how fast they grow) that is at least 2 - 3 standard deviations below what is considered normal.
In less technical terms, this means that if you took 100 children, all of exactly the same age but having a wide range of heights, 1 to 3 of those children might be considered abnormally small (97 of the children are much taller). As far as the growth rate, this can vary slightly depending on the population, race, nutritional condition, etc., but statistical curves can be used to estimate a norm. Again, if you took the same 100 children, all the same age and all from the same population, race, etc., you would find that they're all growing at different rates. If you plotted those rates on a graph, you'd get a curve that looks a bit like a hill. Using a specific mathematical formula, you could then divide the hill in sections centered around the apex (the 50th percentile). If you divide the hill into 100 even sections, these define each percentile. You could then "clump" these percentiles back into groups that define how much they deviate (differ) from the 50th percentile. This is the standard deviation. Most of the population (95%) will fall within 2 standard deviations. About half of the remaining 5% (2.5%) will be growing at a rate that is faster than normal, and the other 2.5% will be growing at a rate that is slower than normal. So, a child who is 2 standard deviations below the norm will be growing at a rate that is less than 97.5% of the population. At 3 standard deviations, the child is growing at a rate that is slower than 99% of the population.
There can be many causes of growth retardation. If your daughter is producing inadequate growth hormone, testing will reveal this and hormone can be supplemented. (The testing is not pleasant, by the way, and many insurance companies will require a lot of testing before agreeing to pay for growth hormone treatments, which is very expensive.) Growth hormone produced during sleep (mostly at night) and is stimulated by vigorous exercise, so make sure your daughter is getting between 10 - 12 hours of sleep each day, with the largest bulk of her sleep being at night. Also, make sure she's getting plenty of exercise daily. Make sure she's not only eating enough, but eating a well balanced diet. This can be difficult to achieve in today's world of processed foods, so a good multivitamin can help bridge any gaps. Maintaining adequate calcium and vitamin D levels are essential for bone growth.
Other abnormal things that can cause growth retardation include kidney or liver disease, and a variety of autoimmune disorders. Inflammatory bowel disease can also cause growth retardation, and often the first sign of Crohn's disease in a child will actually be a growth slow down or stoppage.
Good luck. Our son was diagnosed with idiopathic growth hormone deficiency at age 11 (no growth between ages 9 - 11) and spent 6 years on growth hormone therapy. When he was 13, he showed the first signs of Crohn's disease, which wasn't correctly diagnosed until he was 19 (the growth hormone was partially protective). If you ever need any more advice or need someone to talk to from someone who's "been there", please drop me an email: ____@____.com