Advertising Disclaimer »
Tools and Links
Reader comments are editor-reviewed before posting. See our author guidelines at http://bit.ly/1qJSFq5
More information about text formats
To the Editor,
We write in response to the eLetter from Dr. Enrico Bertino and
colleagues  on the topic of our recent article entitled New Preterm
Infant Growth Curves Based on United States Data. We appreciate the
thoughtful review of this paper. The authors have made several noteworthy
points and we appreciate the opportunity to address their concerns.
First, Dr. Bertino et al. point out that...
First, Dr. Bertino et al. point out that intrauterine growth cannot
be derived from birth data. We agree entirely and made this point in the
article. We agree that premature neonates may not represent a normal
sample of healthy, term infants, however measurement of head circumference
and length in the fetus is not precise.[3, 4] In the absence of accurate
methods for assessing intrauterine growth, we argue that the data
presented are an approximation of fetal growth and are representative of
infants who are admitted to neonatal intensive care units (NICUs) in the
U.S. Moreover, while the absolute centiles may be shifted down compared to
the normal fetus,[5, 6] the slopes (growth velocity) should be similar and
the new curves represent an improvement over what is currently available
in the United States.
We also considered the authors’ suggestion that the growth charts
represent a growth “reference” rather than a “standard”. A reference curve
is “descriptive” of a population whereas a standard is “prescriptive”.
While accurate intrauterine growth data for healthy, term infants would be
the ideal basis for a growth “standard”, these data are not available as
noted above. We based our curves on a large, multi-center, contemporary
sample of infants admitted to NICUs, and excluded infants with birth
defects, multiple births and infants who died prior to discharge. While
this sampling strategy does not assure an ideal sample on which to base a
growth “standard”, it is a significant improvement over previously
published curves and we refer to them in our discussion as “standard-type”
growth curves. We agree that our charts could be more precisely referred
to as “neonatal anthropometric charts.”
Bertino et al. disagreed with our interpretation that “…the size of
our term infants were similar to those in the 2000 Centers for Disease
Control and Prevention’s growth charts”. As they point out, it is
difficult to make direct comparisons because of the differences in the way
the data from the two sources are presented. We shared these same
concerns, and therefore described the size of our term infants as being
similar rather than the same as the CDC 2000 curves. In the development of
the CDC 2000 curves, infants of different gestational ages were pooled
making more direct comparisons difficult. We included 41 week neonates
that were admitted to NICUs, which created a potential selection bias by
including neonates who are not growing well in utero. This is an important
issue raised in response to the commonly used Fenton curves – when should
one start using the CDC 2000 curves and stop using the “neonatal
The commentary raised the important question of race and gender
differences in anthropometric outcomes. We carefully considered the need
for gender and race specific charts. Group-specific charts are important
when there is thought to be a fundamental difference in growth potential
between groups that is not influenced by nutrition, access to or use of
health care, and other adverse environmental influences. We propose that
gender differences in birth size reflect fundamental differences between
boys and girls that truly reflect growth potential. Accordingly, we
presented sex-specific charts. In contrast, we were not confident that the
observed race / ethnic differences reflected genetic potential and could
not be attributed to adverse environmental effects. In addition, race and
ethnicity can be difficult to use clinically because of the increasing
frequency of mixed race children and the wide variability in population of
origin in groups that are broadly defined by the race and ethnic
categories used. In keeping with the policy chosen by the U.S. Centers for
Disease Control in the creation of the CDC 2000 growth charts, we chose to
combine all race and ethnic groups. Since the race and ethnic composition
of our large national sample was comparable to the overall composition of
the U.S. population, we argue that the sample is representative of infants
born in NICUs in the U.S.
Bertino et al. also point out the difficulty in interpreting the
complex formula for calculating Z-scores based on the LMS method. We
published the equation as originally described by Dr. Cole. We
appreciate the clarification that the reviewers provided in how to utilize
this complex formula to compute a Z-score.
Lastly, Bertino et al. raised concern over the irregular shape of the
weight-for-age curve (lower centiles) between 34 and 36 weeks, an issue
that we attribute to the nature of our data for both males and females.
Given the notably large size of this dataset, it is unlikely that the
shape of the curve at these particular ages could be due to sampling or
measurement errors, and we propose that these curves are accurate
reflections of the birth weight of infants in NICUs around the country.
Additionally, the e.d.f. values for the L and S parameters for our weight
curves were 3 and 9 for females and 3 and 8 for males; these values are
very similar to the authors’ recommended e.d.f. values of 4 to 5 and 6 to
8 for L and S, respectively.
We appreciate and value the comments of these esteemed authors.
1. Bertino E, Gilli G, De Curtis M, et al. Some marginal notes on the
new intrauterine growth curves based on United States data[E-letter].
Pediatrics. April 8, 2010.
2. Olsen IE, Groveman SA, Lawson ML, Clark RH, Zemel BS. New
intrauterine growth curves based on United States data. Pediatrics.
3. Lindell G, Marsal K. Sonographic fetal weight estimation in
prolonged pregnancy: comparative study of two- and three-dimensional
methods. Ultrasound Obstet Gynecol. 2009;33(3):295-300.
4. Marsal K, Persson PH, Larsen T, Lilja H, Selbing A, Sultan B.
Intrauterine growth curves based on ultrasonically estimated foetal
weights. Acta Paediatr. 1996;85(7):843-848.
5. Cooke RW. Conventional birth weight standards obscure fetal growth
restriction in preterm infants. Arch Dis Child Fetal Neonatal Ed. May
6. Reeves S, Bernstein IM. Optimal growth modeling. Semin Perinatol.
7. Cole TJ, Green PJ. Smoothing reference centile curves: the LMS
method and penalized likelihood. Stat Med. 1992;11(10):1305-1319.
TO THE EDITOR
In a recent paper Olsen...
In a recent paper Olsen et al. present
“new intrauterine growth curves based on United States data” . This interesting paper provides an updated
picture of the anthropometric characteristics of a large sample of neonates
admitted to NICUs in 33 states; but, in our opinion, some statements may be
misleading for the reader.
intrauterine growth over time cannot be derived from birth data, as the Authors
themselves admit. To avoid confusion,
the expression neonatal anthropometric
charts is preferable, because intrauterine
growth curves can only be based on fetal serial measurements . Also, the Authors state that their charts,
based on measures taken at birth on infants admitted to NICUs “do represent
intrauterine growth of fetuses up to the time of birth, which is the American Academy of Pediatrics recommended goal
for preterm infants”. In fact, the goal
declared by the AAP is to obtain “a rate of growth approximating that of the
third trimester of intrauterine life” .
Such a rate of growth, however, cannot be deducted
by the size attained by preterm infants who “at birth are
smaller in size than fetuses of the same GA”.
Second, the charts are presented as a standard, i.e. a norm that indicates how growth should be, whereas
they are a reference, which describes
how growth actually is. This is a not
negligible distinction [4, 5]. Actually,
it is difficult to believe that term neonates that require admission to NICU
are free from pathologies affecting fetal growth. In any case, the inclusion/exclusion criteria
for the construction of a growth chart should be precisely defined.
Third, the Authors write that “the size of our term infants were similar
to those in the 2000 Centers for Disease Control and Prevention’s growth
charts”, but this statement is not adequately supported. It is hard to compare data conditional on GA
(this study) with the 2000 CDC charts, which pool together all birth data of
neonates weighing 1500 g
or more, irrespectively of their GA. As
the table shows, up to 39 weeks the USA 2010 median lies below the 2000 CDC
median. In absolute terms, the median
birthweight of all 37-41 weeks USA 2010 neonates is about 150 grams below the 2000
CDC median birthweight.
Centile of the 2000 CDC birthweight distribution where the median weight
of USA 2010 neonates lies.
Fourth, in a previous paper, also based on Pediatrix Medical Group
database, Thomas et al.  found
that, on the average, the difference between genders is close to that between
white or Hispanic neonates and black neonates, concluding that there is a need
for “gender and race specific” charts.
Olsen et al. do not explain
why only gender has been taken into account for the construction of the new
Fifth, the LMS parameterization used to trace smooth neonatal charts
allows to transform the values of an auxological trait to z‑scores, even
when the distribution of the trait is skewed.
Unfortunately, the z‑score formula (page e215) is ambiguously
reported as z=[(X/M)L ‑1]/LS, the correct form being z=[(X/M)L
‑1] / (LS). The reader may
ask himself whether he should divide the expression between brackets by the
product L×S, or divide it by L and then multiply by S, according to the
precedence rules. As an example, in the
second case, the z-score of a 41 weeks male born with a weight of 2,755 grams (3rd
centile, z = ‑1.88) would be ‑0.035 (49th centile), and,
vice versa, the z-score of a 41 weeks male born with a weight of 4,633 grams (97th
centile, z = +1.88) would be +0.035 (51st centile).
Sixth and last, the centiles show an irregular shape, mostly evident for
the 3rd and 10th birthweight centiles. This pattern, possibly due to a high number
of equivalent degrees of freedom (e.d.f.) used to smooth L(t) and S(t)
functions (the Authors did not give the e.d.f. of these functions), seems to
have no plausible explanation: e.g. why ever the slope of these centiles
decreases between the 34th and 36th week of
et al , who used the LMS method to
trace the anthropometric charts of Scottish neonates, found that 4
or 5 e.d.f. are enough to smooth
the shape of the L(t) values, and 6 or 8 e.d.f. to smooth the S(t) values. A reanalysis of the data coming from the
Italian Neonatal Study (INeS) and used to trace the neonatal charts for Italy 
showed that no significant decrease in residual deviance is attained by
increasing the number of e.d.f. above 4 (L(t)) and 7 (S(t)). We believe that, even when the LMS model is
used simply as a tool to embellish unattractive shapes, the principle of
parsimony should not be neglected.
(1) SCDU Neonatologia, Dipartimento di Scienze Pediatriche, Università
degli Studi di Torino.
(2)Dipartimento di Pediatria, Università di Roma “La Sapienza”.
(3)Sezione di Statistica Medica e Biometria “GA Maccacaro”, Università
degli Studi di Milano.
1. Olsen IE, Groveman SA, Lawson ML, Clark RH, Zemel BS. New intrauterine growth curves based
on United States
data. Pediatrics. 2010;125: e214–e224
Bertino E, Milani S, Fabris C, De Curtis M. Neonatal anthropometric charts: what
they are, what they are not. Arch Dis Child Fetal Neonatal Ed. 2007; 92: F7-F10
Academy of Pediatrics,
Committee on Nutrition. Nutritional
needs of Low-Birth-Weight infants. Pediatrics. 1977; 60: 519-530
Grummer-Strawn LM, Ogden CL, Mei Z, Flegal KM, Johnson
CL, Kuczmarski RJ, Curtin LR. Scientific and pratical issues in the development of
the US Childhood Growth Reference. In:
Martorell R, Haschke F, eds. Nutrition and growth. Philadelphia:Lippincott Williams & Wilkins. 2001: 21-33
5. Cole TJ.
Growth reference standards. In:
Nicoletti I, Benso L, Gilli G, eds.
Physiological and pathological auxology. Firenze: Edizioni Centro StudiAuxologici. 2004: 161-174
Thomas P, Peabody J, Turnier V, Clark RH. A new look at intrauterine growth and
the impact of race, altitude, and gender. Pediatrics. 2000;106(2). Available at: www.pediatrics.org/cgi/content/full/106/2/e21
7. Bonellie S, Chalmers J, Gray R, Greer I,
Jarvis S, Williams C. Centile charts for
birthweight for gestational age for Scottish singleton births. BMC
Pregnancy Childbirth. 2008; 8: 5-14
Bertino E, Spada E, Occhi L, Coscia A, Giuliani G, Gagliardi L, Gilli G,
Bona G, Fabris C, De Curtis M, Milani S.
Neonatal Anthropometric Charts: the Italian Neonatal Study (INeS)
compared to other European studies.JPGN J Pediatr Gastroenterol Nutr. 2010 (in
Thank you for your interest in spreading the word on Pediatrics.
NOTE: We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail. We do not capture any email address.