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Body Composition of Newborn Twins: Intrapair
Differences
Belinda Koo MAa, Jocelyn Walters MSa, Elaine Hockman PhDc & Winston Koo MBBSab
a Departments of Pediatrics, Obstetrics and Gynecology, University of Tennessee,
Memphis, Tennessee (B.K., J.W., W.K.)
b Department of Pediatrics, Hutzel Hospital (W.K.), Detroit, Michigan
c Computing and Information Technology, Wayne State University (E.H.), Detroit,
Michigan
Published online: 24 Jun 2013.
To cite this article: Belinda Koo MA, Jocelyn Walters MS, Elaine Hockman PhD & Winston Koo MBBS (2002) Body
Composition of Newborn Twins: Intrapair Differences, Journal of the American College of Nutrition, 21:3, 245-249, DOI:
10.1080/07315724.2002.10719217
To link to this article: http://dx.doi.org/10.1080/07315724.2002.10719217
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Original Research
Body Composition of Newborn Twins:
Intrapair Differences
Belinda Koo, MA, Jocelyn Walters, MS, Elaine Hockman, PhD, Winston Koo, MB BS
Departments of Pediatrics, Obstetrics and Gynecology, University of Tennessee, Memphis, Tennessee (B.K., J.W., W.K.),
Department of Pediatrics, Hutzel Hospital (W.K.), Computing and Information Technology, Wayne State University (E.H.),
Detroit, Michigan
Key words: body composition, neonates, twins, growth, bone, lean tissue, fat
Objective: To measure body composition in newborn twins and to test the hypothesis that differences in
body weights between twins are reflected proportionally by their differences in various components of body
composition.
Methods: 48 pairs of newborn twins delivered at a tertiary teaching hospital had dual energy x-ray
absorptiometry (DXA) body composition measurement for bone mineral content (BMC), lean and fat mass (LM,
FM). Data analyzed with regression and analysis of variance.
Results: Body weight, BMC, LM and FM increased with increased gestational age (p Ͻ 0.001). The percent
difference in BW between each twin pair was significantly correlated with percent difference in BMC, LM, and
FM (p Ͻ 0.001). However, mean (Ϯ SD) percent difference in body weight (14.3 Ϯ 10.0%) was significantly
lower (p Ͻ 0.001) than FM (26.0 Ϯ 15.0%) but was not significantly different from LM (13.4 Ϯ 9.0%) or BMC
(15.9 Ϯ 11.6%).
Conclusion: In newborn twins, body weight and body composition varies with gestational age. For any twin
pair, a difference in body weight was correlated with but not proportional to differences in individual
components of body composition.
weights between twins are reflected proportionally by differences
in various components of body composition, specifically lean
INTRODUCTION
Discrepant fetal growth in twin pregnancy is well known. It
has been defined as the difference in birth weight between
twins of greater than 15% [1], 20% [2], 30% [3] or more [4,5].
Its presence is associated with increased risk of perinatal mor-
bidity and mortality [3–6] and long-term physical and intellec-
tual impairment [4]. However, other than standard anthropo-
metric and fetal ultrasound measurements [2,7], there has been
no systematic study on the relative contribution of different
components of body composition to the body weight of twins.
Information on body composition of twins may contribute to
the knowledge on physiological changes during normal and ab-
normal fetal growth. It also may lead to better postnatal nutritional
management of the growth impaired twin if the goal is to achieve
body composition similar to the normally grown twin. This study
aims to determine the variations in anthropometric and body
composition measurements of newborn twins at different gesta-
tional ages and to test the hypothesis that differences in body
body mass, fat mass and bone mineral content.
SUBJECTS AND METHODS
Subjects
Subjects included 48 pairs of twins with birth weights from
976 g to 3135 g and gestation ages from 30 to 40 weeks.
Gestational age assessment was based on menstrual and/or
ultrasound dating and confirmed by standard physical exami-
nation [8]. Seven pairs of twins were Ͻ33 weeks gestational
age, 29 pairs were between 33 and 36 weeks, and 12 pairs were
Ն37 weeks. In 29 pairs, both twins had birth weights appro-
priate for gestational age (AGA, between 10th to 90th percen-
tiles) [9]; in one pair, both twins were small for gestational age
(SGA, less than 10th percentile); and in 18 pairs, one twin was
Address correspondence to: Dr. Winston Koo, Department of Pediatrics, Hutzel Hospital, 4707 St Antoine Blvd, Detroit, MI 48201. E-mail: wkoo@wayne.edu.
Journal of the American College of Nutrition, Vol. 21, No. 3, 245–249 (2002)
Published by the American College of Nutrition
245

Body Composition of Twins
AGA while the other twin was SGA. There were 23 pairs of
twins with a difference in birth weight of greater than 15%.
Thirty-seven pairs of twins were African American (38 males
and 36 females; 21 pairs were of the same gender), 10 pairs
were Caucasian (14 males and 6 females; 8 pairs were of the
same gender), and one pair was Asian (1 male and 1 female).
Clinical care of all study subjects was managed by the
attending physician, and all infants were clinically well at the
time of study. There was no congenital malformation or spe-
cific conditions other than that related to multiple fetuses to
account for the growth discrepancy within each twin pair. This
study was approved by the Institutional Review Board for
human subjects at the University of Tennessee-Memphis, and
written informed consent was obtained from each subject’s
parent.
repeated measurements of spine phantom were Ͻ0.3% for all
parameters. The average annual rate of change for each of these
measurements was not significantly different from zero. In our
laboratory, the in vivo replication of DXA measurements in 50
infants (17 infants were neonates with weights between 1525g
and 5128g) was highly significantly correlated (r Ն 0.99 and
p Ͻ 0.001 for all parameters, i.e., bone mineral content, bone
area, bone mineral density, lean body mass and total fat mass),
and the standard deviation of difference [12] between paired
DXA measurements for the same parameters was 3.8%, 2.5%,
2.6%, 2.3% and 7.0%, respectively.
Statistical Analyses
The difference in weight, length and each measured DXA
variable (lean body mass, fat mass, bone mineral content, bone
area) between each twin pair was expressed as a percentage of
the larger twin with the formula [(larger infant—smaller in-
fant)/(larger infant)] ϫ 100. Bone mineral density was used
only as descriptive data and was not analyzed statistically
because it is based on bone mineral content divided by area and
there are a number of concerns with its use in pediatrics [13].
Regression analysis was used to determine 1) whether the
percent difference in birth weights or body composition com-
ponents listed above was related to gestational age and 2) the
change in anthropometric and DXA measurements with in-
creased gestational age for individual infants. Pearson correla-
tion was used to determine the relationship among percent
differences in nude weight, lean body mass, fat mass and bone
mineral content.
Anthropometric Measurements
Nude weight, length and head circumference of each infant
was measured immediately preceding dual energy x-ray ab-
sorptiometry (DXA) study. Weight in grams was determined
with a digital electronic scale (Air Shields Vickers, Hatboro,
PA). The scale was regularly maintained by the hospital Bio-
medical Instrumentation personnel and calibrated with known
standard weights. Recumbent length was the average of two
consecutive measurements within 0.4 cm and was determined
using a standard length board (Ellard Instrumentation Ltd.,
Seattle, WA). Head circumference was the average of two
consecutive maximum occipitofrontal circumferences within
0.2 cm using a disposable paper tape measure.
Repeated measure analysis of variance was used to test the
premise that the percent difference in nude weight between
twins would be equal to the percent difference of each weight
component of body composition, namely lean body mass, fat
mass and bone mineral content. The dependent variables were
percent difference between each twin pair in nude weight, lean
body mass, fat mass and bone mineral content, i.e., the four
percent differences would be statistically equivalent based on
testing our hypothesis with repeated measures analysis of vari-
ance. The analysis was controlled for race (African American
vs. non-African American) and gender (same sex vs. different
sex). Repeated measures analysis of variance was then repeated
with each within-subject factor adjusted for DXA measured
area. Orthogonal contrasts using Difference and Helmert meth-
ods were used to test for differences among nude weight and
body composition variables.
To further explore the relation of gender to differences in
weight and body composition, the gender variable was further
divided into four categories for each twin pair (i.e., both males,
both females, male weighing more than female, female weigh-
ing more than male), and the same statistical procedure was
repeated. In addition, the Bonferroni test was used for post hoc
comparison among the four categories of the gender variable.
Power was computed for the analyses completed. All statistical
DXA measurements
DXA whole body scans were performed at a mean of 3.8 Ϯ
3.2 (SD) days after birth using a pencil beam densitometer
(Hologic QDR 1000/W. Hologic Inc., Waltham, MA). Forty-
two pairs of twins were studied on the same day. Six pairs of
twins were studied at one to four days apart while awaiting
recovery from minor illnesses in one of the twins.
Details of DXA measurements have been reported [10,11].
Briefly, all scans were performed with the subject and a step
phantom placed on top of an infant platform with an interpos-
ing cotton blanket. Each subject was swaddled in another
cotton blanket during scanning. All infants were scanned with-
out sedation or additional restraint. Each scan was judged
technically satisfactory if the external calibration step phantom
and the skeletal outline of the subject laid within the scan
region as shown on the video monitor and if there was no
significant movement artifact [11]. Scan analysis was per-
formed using the software developed in conjunction with the
manufacturer (Version V5.64P).
Quality control scans were performed daily on a manufac-
turer-supplied anthropomorphic spine phantom, and the long
term (Ͼ3 years) coefficients of variation for the determination
of bone mineral content, bone area and bone mineral density on
246
VOL. 21, NO. 3

Body Composition of Twins
tests were performed with SPSS 10.0 (SPSS Inc., Chicago, IL)
for windows at an adopted significance level of p Ͻ 0.05.
RESULTS
There was no significant relation between the magnitudes of
intrapair difference in body weight or body composition (lean
body mass, fat mass, bone mineral content, bone area) with the
gestational age (adjusted r2 Յ 0.02 for all comparisons). An-
thropometric and DXA measurements of individual subjects
were significantly (p Ͻ 0.001 for all comparisons) higher with
increased gestational age (Figs. 1 and 2). With increasing
gestational ages, there was an increase in bone mineral content
and fat mass, but a decrease in lean body mass as a percentage
of the body weight.
The intrapair percent difference in nude weight was signif-
icantly (p Ͻ 0.001 for all comparisons) correlated with percent
difference in each of the body composition components: lean
body mass (r ϭ 0.93), bone mineral content (r ϭ 0.75) and fat
mass (r ϭ 0.55). However, the percent difference in fat mass
Fig. 2. Scatter plot including mean regression line for body composi-
tion measurements of all study infants (n ϭ 96) as a function of
gestational age (GA) in weeks. Lean body mass g ϭ Ϫ2179 ϩ 116.49
GA, r ϭ 0.76, p Ͻ 0.001. Fat mass g ϭ Ϫ574 ϩ 23.27 GA, r ϭ 0.56,
p Ͻ 0.001. Bone mineral content (BMC) g ϭ Ϫ70.3 ϩ 3.09 GA, r ϭ
0.72, p Ͻ 0.001.
was significantly higher than the percent difference in nude
weight, lean body mass and bone mineral content (Table 1).
There was a tendency for an interaction between percent
difference in body composition and race (p ϭ 0.056, repeated
measures analysis of variance) with African American twins
having a lower percent difference in lean body mass (p Ͻ 0.05,
repeated measures analysis of variance with contrast). How-
ever, the racial effect was no longer present after adjusting each
dependent variable by DXA bone area.
There was no gender effect, whether it was categorized as
same or different gender pairs, or in the four combinations of
gender pairing as described above. In addition, no interaction
effect among the dependent variables with race or gender was
present in any analyses.
From a prospective aspect with the assumption that, for any
twin pregnancy, the larger and presumably normally grown
twin has similar body composition to that of a singleton [14,15]
and the smaller twin would have discrepant body composition,
Fig. 1. Scatter plot including mean regression line for anthropometric
measurements of all study infants (n ϭ 96) as a function of gestational
age (GA) in weeks. Nude weight g ϭ Ϫ2736 ϩ 137.97 GA, r ϭ 0.75,
p Ͻ 0.001. Length cm ϭ 21.7 ϩ 0.63 GA, r ϭ 0.65, p Ͻ 0.001. Head
circumference (HC) cm ϭ 12.3 ϩ 0.55 GA, r ϭ 0.64, p Ͻ 0.001.
JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION
247

Body Composition of Twins
Table 1. Percent Difference* in Anthropometric and Dual
Energy X-Ray Absorptiometry (DXA) Measurements for the
48 Pairs of Twins
body weight is a major physiological predictor of body com-
position during infancy. In this study, the data were analyzed as
a continuum to better reflect fetal growth rather than arbitrarily
defining discrepant fetal growth based on appropriateness of
the birth weight for gestational age of each infant or a 15% to
30% difference in body weight. The use of arbitrary grouping
may have some value for the prediction of clinical course, but
it lacks specificity for use in individual infants. It also ignores
errors of measurement with loss of data and loss of power
particularly around the cut-off point [22]. The magnitude of
differences in body weight and body composition variables
(lean body mass, fat mass, bone mineral content and bone area)
were not related to gestational age; thus further analysis based
on stratification by gestational age would not be justified.
We controlled for race and gender in our data analyses,
since race had a small influence on DXA bone mass measure-
ments based on univariate analyses [14] and females had more
fat mass and less lean body mass than males [15]. The racial
effect showing lower percent difference in lean body mass
compared to the differences in fat mass and bone mineral
content was statistically insignificant once we controlled for
DXA bone area, suggesting that body size is of greater impor-
tance in determining body composition. Our data also demon-
strated that gender pairing did not have an effect on body
composition among twins. This would support the conclusion
that growth and body composition within any twin pair are
independent of gender.
Anthropometric Measurements
Mean Ϯ SD
Range
Study Head Circumference
Study Length
Nude weight
4.0 Ϯ 3.5
4.3 Ϯ 3.5
14.3 Ϯ 10.0
0–16
0–15.9
0.1–42.2
DXA Measurements
Bone mineral density
Bone area
Lean Body Mass
Bone mineral content
Fat Mass
7.2 Ϯ 6.0
11.2 Ϯ 7.7
13.4 Ϯ 9.0
15.9 Ϯ 11.6
26.0 Ϯ 15.0†
0–24.2
0.1–35.1
0.1–31.1
0.4–45.0
0.2–67.7
* Presented in ascending order for difference in overall mean difference.
† Percent difference in fat mass is higher than that for nude weight, lean and bone
mass, p Ͻ 0.001 repeated measures analysis of variance with contrasts using
Helmert and Difference methods.
then a sample size of 40 pairs of twins is expected to detect a
minimum of 15% difference in at least one of the body com-
position components with an ␣ of 0.05 and a power of 0.71,
whereas 50 pairs would increase the power to 0.80. Post hoc
calculation shows that the observed power was Ͼ0.90 for all
analyses.
DISCUSSION
This study employed the DXA technique because of its
ability to simultaneously measure multiple components of body
composition and the increasing availability of normal data
during infancy [16]. Validation studies for the current version
of software have been performed on piglets as small as 886g
[10,17,18], and it is generally agreed that DXA measurements
better reflect the chemical determination of lean and bone
masses compared to fat mass [17,18]. However, the stability
and precision of each DXA parameter of body composition
measurements in our laboratory indicate that our data provided
accurate discriminative power for differences in body compo-
sition between subjects; that is, clinically relevant parameters
of body composition for the management of growth retarded
twins can be determined using this technique.
To our knowledge, this is the first report on the relative
contribution of several components of body composition in-
volving soft tissues (fat and lean body mass) and bone in twins.
For all infants, the pattern of changes in anthropometric and
body composition measurements with increased gestational age
are consistent with that reported for normal singleton infants
[14,15], specifically, an increase in absolute values for all
measurements, but a small decrease in lean body mass as a
percentage of total weight.
The lack of detail data on placental anatomy for our subjects
limits the interpretation on the role of placental circulation that
might account for the altered growth and body composition.
The absence of specific conditions such as congenital malfor-
mation that affected only one fetus of a twin pair suggest that
any adverse in-utero event that may affect growth and body
composition could affect both twins, although the susceptibility
to adverse events might differ within and between twin pairs. In
any case, our data on the differences in body composition
among twin pairs are consistent with the reports in singleton
infants that body fat is most frequently affected and to the
greatest extent by abnormal fetal growth. This appears to be the
case whether the fetal growth was impaired [23–25] or was
excessive [26]. We have now demonstrated that the deficit in
body fat is also greater than the deficit in lean or bone mass in
the smaller or growth retarded twin compared to the larger
twin. This alteration in body composition appears to be the case
in the range of differences in body weights (up to 42%) among
the twin pairs studied.
In this study, body weight is disproportionately affected to
a greater extent than length or head circumference. This finding
supports the presence of asymmetric growth retardation, an
indication of nutrient deficiency as the primary cause of growth
discrepancy, in contrast to the more uniform decrease in
weight, length and head circumference that would be expected
from other causes such as chromosome abnormalities or severe
We used differences in body weight for comparison with
differences in body composition because body weight is an
accurate and reproducible measurement and is freely available.
In addition, we [14,15,19] and others [20,21] have shown that
248
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Body Composition of Twins
intrauterine infection. While a greater intake of energy is usu-
ally recommended for growth impaired infants, it is important
to note that the difference in body weight is also reflected in
differences in lean body mass and bone mineral content. Our
findings suggest that an increase in all nutrients with a propor-
tionally greater energy intake is most appropriate for postnatal
“catch up” on all components of body composition.
10. Koo WWK, Massom LR, Walters J: Validation of accuracy and
precision of dual energy x-ray absorptiometry for infants. J Bone
Mineral Res 10:1111–1115, 1995.
11. Koo WWK, Walters J, Bush AJ: Technical considerations of dual
energy x-ray absorptiometry based bone mineral measurements for
pediatric studies. J Bone Mineral Res 10:1998–2004, 1995.
12. Bland JM, Altman DG: Statistical methods for assessing agree-
ment between two methods of clinical measurement. Lancet
1:307–310, 1986.
13. Nelson DA, Koo WWK: Interpretation of absorptiometric bone
mass measurements in the growing skeleton: issues and limita-
tions. Calcif Tiss Internat 65:1–3, 1999.
CONCLUSION
Anthropometric and body composition measurements of
twins increased with advancing gestational age. However, dif-
ference in body weights between neonates for any twin pair is
correlated with but is not proportional to differences in indi-
vidual components of body composition. Lean body mass and
bone mass are relatively better preserved than fat mass in the
growth impaired twin, and these changes are independent of
race or gender.
14. Koo WWK, Walters J, Bush AJ, Chesney RW, Carlson SE: Dual
energy x-ray absorptiometry studies of bone mineral status in
newborn infants. J Bone Miner Res 11:997–1002, 1996.
15. Koo WWK, Walters JC, Hockman EM: Body composition in
infants at birth and postnatally. J Nutr 130:2188–2194, 2000.
16. Koo WWK: Body composition measurements during infancy. Ann
NY Acad Sci 904:383–392, 2000.
17. Picaud JC, Rigo J, Nyamugabo K, Milet J, Senterre J: Evaluation
of dual energy X ray absorptiometry for body composition assess-
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18. Fusch C, Slotboom J, Fuehrer U, Schumacher R, Keisker A,
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ACKNOWLEDGEMENT
Supported by a University of Tennessee Medical Research
Grant and by The University of Tennessee—Memphis Clinical
Research Center, USPHS Grant RR 00211-29.
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