The main difficulty in designing a preterm standard charts is that it is difficult to define an ideal healthy preterm.
2013 Fenton chart is upgraded form of Fenton 2003 fetal infant growth chart. The new charts have specific curves for male and female and have been tuned to match WHO growth charts after 50 weeks. Other specialty is the charts are design to allow plotting between the weeks rather than completed weeks and intervals have also divided in between 500 gram intervals.
Below are the Charts published in BMC 2013 by T Fenton.
Special tool is available at http://peditools.org/fenton2013/
These growth charts are not standard or a prescription to how infant should grow but they are rather a descriptive approach of how preterm infants grow.
Fenton 2013 have vastly replaced the old charts like Lubchenco, Babson Benda, Erenkranz and other charts for preterm infants.