One of the formula companies distributes a premie growth chart.
It's adapted from BabsonSG & Benda GI: Growth graphs for the
clinical asssessment of infants of varying gestational age. _J
Pediatr 89:814-820, 1976.
You could also go back to any texts you may have access to
that have info about fetal development.

The concept of "adjusted age" is very useful in dealing with premie
growth and development.
Take the number of weeks the baby was early away from the normal
expected gestation at birth (40 weeks).  Now you have a "xx weeker."
Then you begin to add back on the number of weeks since the birth
to get adjusted age.  For example, a baby who was born 6 weeks early
(34 weeker) who is now 2 weeks old would be  36 weeks adjusted age
(expected to be the size and developmental equivalent of a baby who
is
36 weeks or 4 weeks premature still).  By the time that same baby is
10 weeks old, we'd expect about typical 1 month old size and
capabilities.

In years of working and teaching about premies, i've found there's
no rule about how long you have to deal with "adjusted age".  A few
years ago, when survival was not expected before 27-28 weeks, many
babies
were caught up to their chronoligical age (ie. had made up for being
premie) by 2-3 years.  Now, with babies surviving who were 15 weeks
early/
25 weekers, I expect it takes them longer.  And those who are only a
few weeks
early (less than 6), you might not be able to tell by the end of the
first
6-12 mos.

HOWEVER, and this is the big issue:  it really depends alot on why
they were premie
and what happened to them biologically/medically during the initial
recovery weeks.
Babies who were sicker or SGA and premie tend to have a harder time
recovering.

Premies and development are some of my favorite topics, can ya tell?!-
-Phyl