REVIEW OF THE FINDINGS AND METHODOLOGY IN THE STUDY OF
GESTATIONAL WEIGHT GAIN AND BIRTHWEIGHT IN TWINS
by
Sarah Jean Pugh
B.S., The Pennsylvania State University, 2011
Submitted to the Graduate Faculty of
Epidemiology
Graduate School of Public Health in partial fulfillment
of the requirements for the degree of
Master of Public Health
University of Pittsburgh
2013
UNIVERSITY OF PITTSBURGH
GRADUATE SCHOOL OF PUBLIC HEALTH
This essay is submitted
by
Sarah J. Pugh
on
April 16, 2013
and approved by
Essay Advisor:
Lisa Bodnar, PhD, MPH, RD
Associate Professor
Department of Epidemiology
Graduate School of Public Health
University of Pittsburgh
Essay Reader:
Katherine Himes, MD
Assistant Professor
Department of Obstetrics, Gynecology and Reproductive Sciences
School of Medicine
University of Pittsburgh
ii
Copyright © by Sarah J. Pugh
2013
iii
Lisa Bodnar, PhD, MPH, RD
REVIEW OF THE FINDINGS AND METHODOLOGY IN THE STUDY
OF GESTATIONAL WEIGHT GAIN AND BIRTHWEIGHT IN TWINS
Sarah J. Pugh, MPH
University of Pittsburgh, 2013
ABSTRACT
Objectives: To systematically evaluate findings from the current literature on the association
between gestational weight gain and infant birthweight and ‘ideal’ or ‘optimal’ birth outcome in
twin pregnancies and highlight the methodological limitations in this literature.
Methods: We conducted a systematic review using a combination of MeSH search terms,
including ‘gestational weight gain’ ‘twins’ and ‘birthweight’ using PubMed and Ovid
MEDLINE. Articles were cross-referenced and citations were reviewed from the 2009 Institute
of Medicine (IOM) report, “Weight Gain During Pregnancy: Reexamining the guidelines”.
Fourteen studies were selected for the review based on inclusion criteria of gestational weight
gain as the exposure and infant birthweight as an outcome.
Results: A trend of increasing total, total rate, and trimester specific rate of gestational weight
gain was positively associated with infant birthweight and the likelihood of an ‘ideal’ or
‘optimal’ birth outcome; a combination of birth weight and gestational age, in women who were
underweight and normal weight before pregnancy. Evidence was equivocal in overweight and
obese women.
Discussion: The studies reviewed provide some evidence of an association between increased
gestational weight gain and improved twin birth weight. However, there are major methodologic
limitations with the literature, leading to unclear findings. Future studies should aim to use
gestational weight gain measurements that account for gestational length and incorporate twin
specific measurements such as chorionicity, zygosity, and gender discordance to provide data
needed to inform public health recommendations for optimal weight gain ranges in twin
pregnancies.
iv
TABLE OF CONTENTS
1.0 INTRODUCTION…………………………………………………………………………...1
2.0 METHODS…………………………………………………………………………….……..5
3.0 RESULTS…………………………………………………………………………………….7
3.1 TOTAL GWG AND IDEAL BIRTH OUTCOME………………………………...7
3.2 TOTAL GWG AND BIRTHWEIGHT………………………………………….….8
3.3 RATE OF TOTAL GWG AND IDEAL BIRTH OUTCOME AND
BIRTHWEIGHT…………………………………………...………………………..8
3.4 PATTERN OF GWG AND BIRTHWEIGHT………………………………….…9
4.0 DISCUSSION……………………………………………………………………………….14
4.1 IMPLICATIONS FOR FUTURE RESEARCH………………………………….16
BIBLIOGRPAHY………………………………………………………………………………17
v
LIST OF TABLES
Table 1. Total Gestational Weight Gain and Ideal Outcome in Twin Infants………………..…10
Table 2. Total Gestational Weight Gain and Birthweight in Twin Infants……………………...11
Table 3. Rate of Total Gestational Weight Gain and Ideal Outcome or Birthweight in Twin
Infants…………………………………………………………………………………12
Table 4. Pattern of Gestational Weight Gain and Birthweight in Twin Infants…………………13
vi
LIST OF FIGURES
Figure 1: Flowchart of study selection on gestational weight gain and birthweight in twin
pregnancies………………………………………………………………………………………..6
vii
1.0
INTRODUCTION
The rate of twin births rose 76 percent between 1980 and 2009 from 18.9 to 33.2 twins
per 1,000 total live births (1). Twins account for 3% of all live births, yet represent nearly 1 in 6
infants who die within the first month of life and account for 15% of preterm births (<37 weeks
gestation), 20% of low birthweight (LBW, <2,500 grams), and 19-24% of very low birthweight
(VLBW, <1,500 grams) neonates in the United States (2-7). Compared to singletons, twins are
10 times more likely to be born LBW and VLBW (8, 9).
Birthweight is a measure of fetal nutrient intake and intrauterine fetal growth. High
birthweight and low birthweight are predictive of mortality, developmental outcomes, and
chronic disease (10, 11). There is a strong positive association between birth weight and neonatal
mortality (<28 days), neonatal intensive care unit (NICU) length of stay, and increased risk of
adulthood hypertension and type 2 diabetes (3, 11, 12). The increased risk of adverse outcomes
associated with twin gestations is attributed to increased growth restriction and premature
delivery due to multiple fetuses (13).
Maternal nutrient intake is a predictor of fetal growth, and gestational weight gain
(GWG) is a measure of nutrient intake that is readily obtained at prenatal care visits. In
conjunction with pre-pregnancy weight status, GWG is a modifiable risk factor for LBW in
singletons (14); however, there are fewer studies on the relationship in twins. It is important to
study GWG among women carrying twin pregnancies because while it is logical to assume
1
increased GWG is associated with multiple gestations, the magnitude of the differences is not
well-known. Maternal GWG has been shown to influence the rate of fetal growth and length of
gestation. Nevertheless, total GWG and patterns of GWG associated with twin birthweight are
understudied, and published results may be biased by inappropriate control for length of
gestation.
Length of gestation has a strong positive correlation with birthweight. Moreover, women
with longer gestations have more time to gain weight. Therefore, accounting for gestational age
(GA) is important when examining the relationship between maternal GWG and infant birth
weight or gestational age (15). Rate of GWG (total GWG divided by the GA) is a measure
researchers have used to attempt to account for this correlation. However, rate of GWG remains
correlated with GA and biases the associations between GWG and gestational-age dependent
outcomes (16).
Fetal growth in twins is affected by zygosity, chorionicity, and gender. Chorionicity
refers to the placental vascular arrangement around the fetus (17). A fetus is surrounded by
membranes connected to the placenta. The inner membrane is the amnion and contains the
amniotic fluid (18). The outer membrane is the chorion. Monochorionic twins share the same
placenta and outer membrane, while dichorionic twins have separate outer membranes and
separate placentas (19). Monochorionic twins are at higher risk of adverse outcomes than
dichorionic twins. Approximately 15-20% develop twin-twin transfusion syndrome (TTTS) (a
disorder where blood is shared unequally between fetuses (20) and another 25% develop
discordant growth (20). Chorionicity may also influence gestational length, thereby affecting
birthweight (21).
2
Zygosity refers to the formation of cells at conception. A single fertilized egg that divides
and becomes two genetically identical fetuses is referred to as monozygotic (identical) twins.
Two separate eggs that are fertilized become dizygotic (fraternal) twins (18). Monozygotic twins
formed after day 3 of fertilization are monochorionic twins. Thus, they are at a higher risk of
fetal growth restriction compared to dizygotic or monozygotic twins that divide before day 3
after fertilization due to the risk of TTTS or unequal placental sharing. (20). Only dizygotic
twins can be gender discordant.
Gender influences both gestational length between twin pregnancies and birthweight
with-in twin pairs (21). Same-sex female twins have a longer gestation and lower combined
birthweight than same-sex male twins, while discordant twins have a higher combined
birthweight and longer gestation than same-sex male twins (21).Currently, there is no evidence
to suggest a difference in GWG by zygosity, chorionicity, and fetal gender. Therefore, the
question remains whether zygosity, chorionicity, and gender modify or confound the relationship
between GWG and infant birthweight in twins.
Pre-pregnancy body mass index (BMI) may modify the association between GWG and
the likelihood of an optimal infant outcome. Women who start pregnancy at a lower weight need
to gain more compared to obese woman to support the growth of two fetuses. Stratification by
pregravid BMI status is appropriate to determine optimal GWG for healthy infant and maternal
outcomes in twins (22). Women should be further stratified into classes of obesity due to the
increasing rate of severe obesity (6.5%) among women of childbearing age and the increased
risks associated with severe obesity (23).
3
Our purpose is to provide a critical review of the literature on GWG and infant
birthweight and an ‘ideal’ or ‘optimal’ birth weight outcome in twins. Conclusions drawn from
this study will be used to recommend future research to address the gaps in the literature.
4
2.0 METHODS
This review focuses on GWG in twin gestations with infant birthweight as the outcome.
An Ovid MEDLINE search was conducted restricting from 1990 to 2012 with combined medical
subject heading (MeSH) terms ‘twins,’ ‘weight gain,’ ‘birthweight’ using the operator ‘AND’
and limiting to human subjects and English language. The search was restricted from 1990-2012
to include only women pregnant during the 1990 and 2010 IOM GWG guideline revisions. The
search returned 21 results. We additionally searched PubMed using the general search term
‘gestational weight gain AND twins’ and a separate search combining general field terms
‘twins,’ ‘birthweight,’ ‘growth,’ and ‘gestational weight gain’ using the operator ‘AND’ and
limited to human subjects and English language. The search returned 30 studies. The 2009
Institute of Medicine (IOM) report, “Weight Gain During Pregnancy: Reexamining the
guidelines” was reviewed for references but no new studies were found (22). In addition, each
selected study was hand searched to cross-reference any relevant studies from which 3 studies
were included (Figure 1).
Inclusion of articles met the following criteria: (1) calculated total or rate of GWG as an
exposure variable (2) examined infant birthweight as an outcome independently or as part of an
“ideal” or “optimal” outcome. Eight studies were selected from the Ovid MEDLINE search and
3 studies were selected from the PubMed search.
5
A table of study characteristics and methodology was constructed based on extracted
information from the relevant 14 articles, including the number of pregnancies, the population,
measurement of GWG, measurement of birthweight, and results.
Ovid MEDLINE
(n=21)
PubMed
(n=30)
Ovid Medline Excluded Studies
Childhood weight gain (n=2)
Maternal Nutrient intake (n=2)
Maternal fat composition (n=2)
Preeclamptic pregnancy (n= 1)
Non- GWG exposure variable (n= 5)
Non- Birthweight outcome (n= 1)
Total excluded (n=13)
PubMed Excluded Studies
Ovid MEDLINE duplicates (n= 5)
Childhood weight gain (n=3)
Maternal Nutrient intake (n=2)
Maternal fat composition (n=1)
Abstract (n=1)
Non-GWG exposure variable (n=10)
Non- Birthweight outcome (n=5)
Selected studies from
literature searches
(n= 11)
Total excluded (n=27)
Hand searched and cross
referenced included articles
(n=3)
Reviewed references in the
2009 IOM report (n= 0)
Included Studies (n= 14)
Figure 1: Flowchart of study selection on gestational weight gain and birthweight in twin pregnancies
6
3.0 RESULTS
A total of 54 studies were identified for possible inclusion. After eliminating 40 studies
with a non-gestational weight gain exposure or non-birthweight outcome, 14 studies were
included in the final review. The studies fell into four main categories based on exposure and
outcome classification: 1) total GWG and ideal birth outcome (n=5); 2) total GWG and infant
birthweight (n=4); 3) rate of total GWG and ideal birth outcome or birthweight (n=3); 4) pattern
of GWG and infant birthweight (n=4). Total GWG was calculated as the difference between
weight at delivery and prepregnancy weight. Rate of total GWG was calculated as total GWG
divided by gestational age at delivery. Pattern of GWG was determined based on the rate of
GWG within specific gestational age windows.
Studies included in this review evaluated pregnancies delivered from 1982-2010. Sample
sizes ranged from 163 to 11,827 twin pregnancies. Gestational age (GA) at delivery varied from
≥28 weeks to 42 weeks dependent on study inclusion criteria.
3.1 Total GWG and Ideal Birth Outcome
Five studies examined the association between total GWG and an ideal birth outcome.
Ideal outcome was defined based on variable ranges of gestational ages at delivery combined
with variable ranges of birthweights (24-28). GA ranged from ≥37 weeks, >37-<42 weeks, 35-38
weeks, and ≥36 weeks and birthweight cut points included both twins weight ≥2500 grams, mean
weight 2500-2800 grams, single infant 3001-3500 grams, and smaller twin >10th% for sex and
7
gestational age. Three studies reported a positive association between total GWG and the
likelihood of an ideal birth outcome among underweight and normal weight women (24, 25, 28).
Among overweight women, some studies reported a positive association (24, 28) while others
found no relationship (25). In studies that did not stratify by prepregnancy BMI category, but
adjusted for it in regression models, one reported a positive association and one found no
association between maternal GWG and ‘ideal’ or ‘optimal’ infant outcome (26, 27). Only one of
these studies reported on zygosity (24). The majority of pregnancies were dizygotic (57%) and
no significant relationship with ‘optimal’ outcome was found (24).
3.2 Total GWG and Infant Birthweight
Four studies examined the association between total GWG and the likelihood of both
infants’ birthweights being >2,500 grams (4, 29-31). When stratified by prepregnancy BMI,
investigators consistently found that increasing GWG was associated with increased birthweight
in underweight (4, 29) and normal weight women (4, 29-31). Among overweight and obese
women, the findings were mixed, with three studies reporting a positive association for
overweight women and the remaining finding no association among obese women (4, 29, 30).
Two studies controlled for chorionicity as a confounder between GWG and infant birthweight
(30, 31). No difference in GWG by chorionicity (87% dichorionic) was reported.
3.3 Rate of Total GWG and Ideal Birth Outcome or Birthweight
Rate of total GWG was positively associated with the likelihood of an ideal outcome for
all women in both studies, adjusted for BMI (27, 32). Ideal outcome was defined as mean infant
birthweight 2500-2800g and 35-38 weeks gestation in one study (27) and as SGA or average for
gestational age (AGA) infants in another study (32). Neither study reported on chorionicity,
zygosity, or gender discordance.
8
One study examined rate of total GWG and the likelihood of delivering both twins
weighing ≥ 2500grams , stratified by BMI (33). There was a positive association between
increasing rate of total GWG in underweight, normal weight and overweight women but not in
obese women. Zygosity, chorionicity, and gender discordance were not reported in this study.
3.4 Pattern of GWG and Birthweight
Four studies examined the association between pattern of GWG and twin infant
birthweight (29, 33-36). Pattern of GWG varied by study, but was generally classified as 0-20
weeks and either 20 weeks to delivery (29, 34-36) or 20-28weeks, and 28weeks to delivery.
Birthweight was defined as sum of twin birthweight or average twin birthweight. There was an
overall positive association between increasing GWG across intervals of pregnancy and
improved birthweight outcome (35). Studies that stratified by prepregnancy BMI reported a
positive association in underweight and normal weight women (29, 34, 36). A trend towards a
positive association in overweight women was reported in two of three studies (34, 36).
However, increasing GWG was not associated with a positive birthweight outcome in obese
women (36). Luke et al. (1997) and (1998) reported on and controlled for gender discordance
and chorionicity as a confounder between pattern of GWG and birthweight. Luke et al. (2003)
reported and controlled for only chorionicity.
9
Table 1: Total Gestational Weight Gain associated with an Ideal Outcome in Twin Infants
Birthweight measurement
Reference
Pederson, (1990)
N
217
Population
Swedish Hospital from
1982-1986
Exposure
Luke, (1993)
1,984
163
924
Chor
Zyg
Underwt
X
Yes
Yes
Yes
Yes
Yes
No
Ideal outcome: 2
living infants with
BW ≥2500g , apgar 2:
7 at 5 minutes, GA
≥37 weeks
Kansas linked birth-death
records from 1980-1986
Total
GWG
87.7% White; 7.9% Black
86% married
60% completed ≤12 years
education
Johns Hopkins Hospital
from 1979-1989.
Ideal Outcome: >37 <42 weeks GA and
one or both infants
BW 3001-3500 grams
Total
GWG
Ideal pregnancy: BW
of smaller twin
>10th% and ≤8 days
stay and 35-38 weeks
GA
No
Total
GWG
Ideal Twin outcome:
mean BW 2500-2800
gms and 35-38wks
Yes
Total
GWG
Ideal outcome:
delivery at ≥36wks
and average twin BW
≥2,500g.
66.3% Black; 33.7% White
Luke, (1996)
GD
Total
GWG
84% white
83% married
76% insured
Brown, (1990)
Outcome
Increasing Total GWG significantly associated
with Ideal Outcome
Normal
Overwt
Obese
Overall
27% <12 years education
Mothers of twins who
attended Twins Days
Festival in Ohio in 1989,
1990, 1991, or 1993
No
100% white
Yeh, (2007)
1,342
Upstate NY electronic birth
certificate data from 19992003
Yes
83.7% White; 11.8% Black;
2.2% Hispanic
63% >High school
education
*GD: Gender Discordance; Chor: Chorionicity; Zyg: Zygosity
10
Yes
Yes
No
Table 2: Total Gestational Weight Gain associated with and Birthweight in Twin Infants
Birthweight measurement
Reference
Lantz, (1996)
N
189
Chu, (2009)
6,345
Fox, (2010)
297
Fox, (2011)
170
Population
Tampa hospital
from 1981-1993
59.3% White, 27%
Black
PRAMS data from
28 US states and
NYC from 20012006
69.1% white; 16.5%
Black; 9.6%
Hispanic
57% ≤12 years
education
Private maternalfetal medicine
practice in New
York from 20052009
Majority White
across BMI strata
and 100% insured
Private maternalfetal medicine
practice in New
York from 20052010
Exposure
Total GWG
Outcome
Birthweight of both
twin infants ≥ 2500
grams
Total GWG
Birthweight of 1
twin infant selected
based on PRAMS
algorithm
Total GWG
Both twins >2,500 g
Total GWG
Both twins >2,500 g
GD
Chor
Zyg
Increasing Total GWG significantly associated
with Increased Birthweight
Underwt
Normal
Overwt
Obese
Overall
Yes
Yes
Yes
No
Yes
Majority White
across BMI strata
and 100% insured
*GD: Gender Discordance; Chor: Chorionicity; Zyg: Zygosity
11
Yes
Yes
No
X
Yes
Yes
No
X
Yes
No
No
Table 3: Rate of Total Gestational Weight Gain associated with an Ideal Outcome or Birthweight in Twin Infants
Birthweight measurement
Reference
Luke, (1996)
N
924
Schwendemann,
(2005)
11,82
7
Gonzalez-Quintero,
(2012)
5,129
Population
Mothers of twins
who attended Twins
Days Festival in
Ohio in 1989, 1990,
1991, or 1993
100% white
High-risk
pregnancies from
U.S Matria
Healthcare database
from 1988- 2002
13.8% Black
80% married
High-risk
pregnancies from
Atlanta, GA
insurance plan from
2004-2010.
Exposure
Rate of GWG
Outcome
Ideal Twin
outcome: mean
BW 2500-2800
gms and 35-38wks
Rate of GWG
Atleast one small
for gestational age
(SGA) infant
compared to two
average for
gestational age
(AGA) infants
Rate of GWG
Both twins >2,500
g
GD
Chor
Zyg
Increasing rate of GWG significantly associated
with Increased Ideal Outcome
Underwt
Normal
Overwt
Obese
Overall
Yes
Yes
*GD: Gender Discordance; Chor: Chorionicity; Zyg: Zygosity
12
Yes
Yes
Yes
No
Table 4: Pattern of Gestational Weight Gain associated with Birthweight in Twin Infants
Birthweight measurement
Reference
Lantz, (1996)
N
189
Population
Tampa hospital
from 1981-1993
Exposure
Early Rate of GWG
(0-20 wks) as <1
lb/wk or ≥ 1lb/wk.
Late rate of GWG
(20 wks-delivery)as
<1.75 lb/wk or
≥1.75 lb/wk
Rate of GWG:
calculated at
intervals <20wks,
20-28wks, 28wks to
delivery
Outcome
Birthweight of both
twin infants ≥ 2500
grams
≥28wks gestation in
Baltimore, Miami,
Charleston, and Ann
Arbor from 19791997
34% White; 20%
White-Latin; 45%
Black; 1% Asian
≥28wks in Ann
Arbor, Charleston,
Baltimore, and
Miami from 19791999
37% White; 19%
Hispanic; 43%
Black; 1% Asian
59.3% White, 27%
Black
Luke, (1997)
646
≥ 28wks gestation in
Baltimore (293),
Miami (277), and
(76) Ann Arbor
Hospitals from
1979-1995
GD
Chor
Sum of twin pair
birthweight
X
X
rate of GWG:
calculated at
intervals <20wks,
20-28wks, 28wks to
delivery
Sum of twin
birthweight
X
X
rate of GWG:
calculated at
intervals <20wks,
20-28wks, 28wks to
delivery
Average twin BW at
36-38wks (2,8502,928g)
Zyg
Underwt
Yes
Yes
Increasing rate of GWG significantly
Associated with Increased Birthweight
Normal
Overwt
Obese
Overall
Yes
No
Yes
Yes
37.3% White,
27.3% Hispanic,
44.5% Black
Luke, (1998)
Luke, (2003)
1564
2,324
X
*GD: Gender Discordance; Chor: Chorionicity; Zyg: Zygosity
13
Yes
Yes
Yes
Yes
No
4.0
DISCUSSION
Overall, studies in mothers carrying twins reported that as total GWG increased so too
did the likelihood of an ideal pregnancy outcome and a higher birthweight in underweight and
normal weight women. Findings were similar when GWG was examined as a rate of total GWG
or pattern of GWG across different gestational age windows. This finding is biologically
plausible due to the increased blood flow, placental tissue, maternal energy expenditure required
for vital processes, and increased caloric intake to support the growth and development of two
fetuses as compared to one. The findings on total, rate and pattern of GWG in overweight and
obese women were equivocal, which may be attributed to the limited number of studies and
small population sizes. Only 8 of the 14 studies included a stratum for overweight women, 5 of
which also included a stratum for obesity with most sample sizes less than 50.
Nevertheless, this literature is significantly limited by the use of measures of GWG (total
rate of gain or rate of GWG) that are correlated with length of gestation (15). Some studies
attempted to limit the confounding effects of gestational age by restricting pregnancies to those
with a narrow range of gestational age at delivery. While this method may reduce the bias, it
may not be eliminated as there may still be a correlation between GWG and gestational length
(15).
Zygosity, chorionicity, and gender may modify the amount of weight gain associated
with an ‘ideal’ or ‘optimal’ infant outcome. Data on these clinical characteristics are rarely
14
reported in published studies. This lack of information about twin development limits the ability
to account for a possible modification or confounding of the GWG-fetal growth association (22).
Prepregnancy BMI is a regularly measured proxy of maternal fat stores. Rates of fetal
growth restriction and fetal overgrowth vary by prepregnancy BMI, and the impact of GWG on
these outcomes in obese compared with lean women differs as well (37). Nevertheless, only 8 of
14 studies presented results stratified by prepregnancy BMI category and 5 of 14 studies that
presented results for all women combined adjusted for the confounding effects of BMI.
Infant birthweight is used as an outcome to determine optimal GWG because BW is a
measure of fetal growth, and appropriate growth is an important determinant of short – and longterm offspring health (38). However, BW is highly correlated with length of gestation and
reflects size at delivery rather than growth throughout gestation. Studies did not consider a
gestational-age adjusted measure of birthweight such as birthweight z-scores. Additionally, there
is no consistently agreed upon measure of ideal birthweight in twin pregnancies. Birthweight
also may be modified or confounded by zygosity, chorionicity, and gender discordance, but were
not considered in most studies.
An additional concern of the current evidence is limited populations studied, resulting in
questionable generalizability. It is important to note Luke et al. contributed 5 of 14 papers
described in this review (of 14 available studies), including 4 of which that used the same
population with subsequent years of observation. Fox et al. contributed 2 studies to the review
with the same population and additional participants from subsequent years. Therefore, some
consistency in findings noted here is due to similar populations being studied rather than across
different populations.
15
4.1 Future Implications
The methodological limitations of the studies reviewed make it difficult to determine a
range of gestational weight gain associated with optimal fetal growth in twin pregnancies among
women in different prepregnancy BMI categories. Future studies should aim to fill gaps in
knowledge by using measurements of GWG that are uncorrelated with gestational duration (16),
improved measures of fetal growth, and studying a wide range of BMI categories including
severely obese women. It may be of value to consider a more holistic approach to pregnancy
health as opposed to strictly measurements of BW and GA as outcome endpoints. This research
is important for public health because GWG is a modifiable factor that may improve birth
outcomes in twin pregnancies.
16
BIBLIOGRAPHY
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Martin J, Hamilton, BE, Ventura, SJ., et al. Births: Final data for 2009. National vital statistics
reports. Hyattsville, MD: National Center for Health Statistics2011 Contract No.: 1.
Goodnight W, Newman R, Society of Maternal-Fetal M. Optimal nutrition for improved twin
pregnancy outcome. Obstetrics & Gynecology. 2009; 114:1121-34.
Mathews TJ, MacDorman MF, Menacker F. Infant mortality statistics from the 1999 period
linked birth/infant death data set. Natl Vital Stat Rep. 2002; 50:1-28.
Chu SY, D'Angelo DV. Gestational weight gain among US women who deliver twins, 20012006. American journal of obstetrics and gynecology. 2009; 200:390 e1-6.
Blondel B, Kogan MD, Alexander GR, Dattani N, Kramer MS, Macfarlane A, et al. The impact
of the increasing number of multiple births on the rates of preterm birth and low birthweight: an
international study. American journal of public health. 2002; 92:1323-30.
Donovan EF, Ehrenkranz RA, Shankaran S, Stevenson DK, Wright LL, Younes N, et al.
Outcomes of very low birth weight twins cared for in the National Institute of Child Health and
Human Development Neonatal Research Network's intensive care units. American Journal of
Obstetrics & Gynecology. 1998; 179:742-9.
Russell RB, Petrini JR, Damus K, Mattison DR, Schwarz RH. The changing epidemiology of
multiple births in the United States. Obstetrics & Gynecology. 2003; 101:129-35.
Luke B. What is the influence of maternal weight gain on the fetal growth of twins? Clinical
obstetrics and gynecology. 1998; 41:56-64.
Chauhan SP, Scardo JA, Hayes E, Abuhamad AZ, Berghella V. Twins: prevalence, problems, and
preterm births. American Journal of Obstetrics & Gynecology. 2010; 203:305-15.
Luke B, Brown MB, Alexandre PK, Kinoshi T, O'Sullivan MJ, Martin D, et al. The cost of twin
pregnancy: maternal and neonatal factors. American journal of obstetrics and gynecology. 2005;
192:909-15.
Boardman JD, Powers DA, Padilla YC, Hummer RA. Low birth weight, social factors, and
developmental outcomes among children in the United States. Demography. 2002; 39:353-68.
Luke B, Brown MB, Alexandre PK, Kinoshi T, O'Sullivan MJ, Martin D, et al. The cost of twin
pregnancy: maternal and neonatal factors. American Journal of Obstetrics & Gynecology. 2005;
192:909-15.
Rao A, Sairam S, Shehata H. Obstetric complications of twin pregnancies. Best Pract Res Clin
Obstet Gynaecol. 2004; 18:557-76.
Viswanathan M, Siega-Riz AM, Moos MK, Deierlein A, Mumford S, Knaack J, et al. Outcomes
of maternal weight gain. Evid Rep Technol Assess (Full Rep). 2008:1-223.
Hutcheon JA, Bodnar LM, Joseph KS, Abrams B, Simhan HN, Platt RW. The bias in current
measures of gestational weight gain. Paediatric and perinatal epidemiology. 2012; 26:109-16.
Hutcheon JA, Platt RW, Abrams B, Himes KP, Simhan HN, Bodnar LM. A weight-gain-forgestational-age z score chart for the assessment of maternal weight gain in pregnancy. Am J Clin
Nutr. 2013.
17
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
Royer H. Separated at Girth: US Twin Estimates of the Effects of Birth Weight. American
Economic Journal: Applied Economics. 2009; 1:49-85.
Hediger M. Twins and Higher Order Multiples. [Presentation]. In press 2012.
Hack KEA, Derks JB, de Visser VL, Elias SG, Visser GHA. The natural course of monochorionic
and dichorionic twin pregnancies: a historical cohort. Twin Research & Human Genetics: the
Official Journal of the International Society for Twin Studies. 2006; 9:450-5.
Barigye O, Pasquini L, Galea P, Chambers H, Chappell L, Fisk NM. High risk of unexpected late
fetal death in monochorionic twins despite intensive ultrasound surveillance: a cohort study.
PLoS medicine. 2005; 2:e172.
Loos RJ, Derom C, Eeckels R, Derom R, Vlietinck R. Length of gestation and birthweight in
dizygotic twins. Lancet. 2001; 358:560-1.
Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington
(DC): National Academy Press; 2009.
Bodnar LM, Siega-Riz AM, Simhan HN, Himes KP, Abrams B. Severe obesity, gestational
weight gain, and adverse birth outcomes. American Journal of Clinical Nutrition. 91:1642-8.
Pederson AL, Worthington-Roberts B, Hickok DE. Weight gain patterns during twin gestation.
Journal of the American Dietetic Association. 1989; 89:642-6.
Brown JE, Schloesser PT. Prepregnancy weight status, prenatal weight gain, and the outcome of
term twin gestations. American Journal of Obstetrics & Gynecology. 1990; 162:182-6.
Luke B, Minogue J, Witter FR, Keith LG, Johnson TR. The ideal twin pregnancy: patterns of
weight gain, discordancy, and length of gestation. American journal of obstetrics and gynecology.
1993; 169:588-97.
Luke B, Leurgans SUE. Maternal Weight Gains in Ideal Twin Outcomes. Journal of the
American Dietetic Association. 1996; 96:178-81.
Yeh J, Shelton JA. Association of pre-pregnancy maternal body mass and maternal weight gain to
newborn outcomes in twin pregnancies. Acta obstetricia et gynecologica Scandinavica. 2007;
86:1051-7.
Lantz ME, Chez RA, Rodriguez A, Porter KB. Maternal weight gain patterns and birth weight
outcome in twin gestation. Obstetrics & Gynecology. 1996; 87:551-6.
Fox NS, Rebarber A, Roman AS, Klauser CK, Peress D, Saltzman DH. Weight gain in twin
pregnancies and adverse outcomes: examining the 2009 Institute of Medicine guidelines.
Obstetrics & Gynecology. 2010; 116:100-6.
Fox NS, Saltzman DH, Kurtz H, Rebarber A. Excessive weight gain in term twin pregnancies:
examining the 2009 Institute of Medicine definitions. Obstetrics and gynecology. 2011;
118:1000-4.
Schwendemann WD, O'Brien JM, Barton JR, Milligan DA, Istwan N. Modifiable risk factors for
growth restriction in twin pregnancies. American journal of obstetrics and gynecology. 2005;
192:1440-2.
Gonzalez-Quintero VH, Kathiresan AS, Tudela FJ, Rhea D, Desch C, Istwan N. The association
of gestational weight gain per institute of medicine guidelines and prepregnancy body mass index
on outcomes of twin pregnancies. American journal of perinatology. 2012; 29:435-40.
Luke B, Gillespie B, Min SJ, Avni M, Witter FR, O'Sullivan MJ. Critical periods of maternal
weight gain: effect on twin birth weight. American journal of obstetrics and gynecology. 1997;
177:1055-62.
Luke B, Min SJ, Gillespie B, Avni M, Witter FR, Newman RB, et al. The importance of early
weight gain in the intrauterine growth and birth weight of twins. American journal of obstetrics
and gynecology. 1998; 179:1155-61.
Luke B, Hediger ML, Nugent C, Newman RB, Mauldin JG, Witter FR, et al. Body mass index-specific weight gains associated with optimal birth weights in twin pregnancies. The Journal of
reproductive medicine. 2003; 48:217-24.
18
37.
38.
Catalano PM. Management of obesity in pregnancy. Obstetrics and gynecology. 2007; 109:41933.
Luke B. Nutritional influences on fetal growth. Clinical obstetrics and gynecology. 1994; 37:53849.
19