I wanted to correct, her progression was:
Birth: 20.75" - 97% (we're in Canada, so we use the WHO Canada charts)
6 weeks: 22.25" - 60-65%
3 months: 23.25 - 45%
6 months: 25.5 - 45%
12 months: 28.5 - 40%
15 months: 30" - 30%
18 months: 31" - 20%
20 months: 31.25" - 15%
24 months: 32.25" - 10%
If you go from birth she has crossed 3 lines, from 6 weeks, 2 lines. I also wanted to add that it seems that her weight has really started going up. At 12 months she was 18.5 lbs, which gave her a weight for length of just shy of 50%, at 18 months she was 20.5 lbs, still around 45%, and now at 24 months, she's 25 lbs (she put on 5 lbs in the last 6 months), which is JUST below the 85% line.
It is good to hear she is gaining weight. She has been through a lot in the short amount of time in her life. If she continues to not grow, suggest having her blood and hormone levels checked. Thank you for your question.