Abstract
Objective
To assess neonatal outcomes according to gestational weight gain (GWG) in twins
Study Design
This was a retrospective cohort study of twins delivered at ≥ 24 weeks. GWG was defined using the IOM guidelines as the referent. Birthweight and NICU admissions were compared with Chi-square and ANOVA tests, stratified by BMI.
Results
In all three BMI groups, mean birthweight of the larger and smaller twin increased as GWG increased, p<0.01. For the underweight/normal weight group, both twins < 2500 g, <1500 g and small for gestational age decreased significantly as GWG increased. Birthweight < 2500 g increased in all groups with GWG below the IOM guidelines, p < 0.01. In the multivariate analysis, both twins < 2500 g was significantly decreased with GWG above IOM guidelines. There was no difference in NICU admissions with GWG above the IOM guidelines.
Conclusion
GWG above the IOM guidelines may improve twin birthweights, with the findings most significant in underweight/normal weight women.
Introduction
In 2009, the IOM revised their guidelines and recommended that normal weight women with a twin gestation gain 17-25 kg.1 They also updated the guidelines for overweight and obese women as prepregnancy BMI influences important outcomes such as premature birth and intrauterine growth restriction.2 For overweight and obese women, the guidelines are 14-23 kg and 11-19 kg, respectively.1 The committee developed these guidelines based on the 25th-75th percentiles of weight gain in women who had twin birth weights > 2500 g at term.1 The committee did not conduct the same rigorous analysis of outcomes by gestational weight gain for twins as it did for singleton gestations, thus leading only to provisional guidelines.
The IOM has different weight gain guidelines for twin compared to singleton gestations for several reasons. Twin gestations have many important factors that affect maternal weight gain including maternal physiologic adaptions to a twin gestation, prepregnancy maternal weight and discordance in birth weight.3 Birth weight in twin gestations differs from singletons, as the 10th percentile of birth weight for singletons is the mean birth weight for twins at 38 weeks gestation.4 Also, twins comprise only 3% of all live births in the United States, but approximately 60% are born preterm and approximately 25% are very low birthweight.5,6
Other studies have suggested that gestational weight gain above the guidelines might improve neonatal outcomes. 7-9 We hypothesized that we would observe improved neonatal outcomes as gestational weight gain increases in our contemporary obstetric population. The objective of this study was to evaluate neonatal outcomes (birth weight, NICU admissions) and maternal outcomes associated with gestational weight gain in twin gestations, stratified by BMI class (underweight/normal weight, overweight, obese), using the 2009 IOM guidelines as a reference.
Materials and Methods
This retrospective cohort study was performed using the Consortium on Safe Labor database. Data were abstracted from an electronic medical record at each contributing institution from 2002-2008. The complete database contained 228,438 deliveries from twelve clinical centers from nineteen distinct hospitals across nine ACOG districts. Detailed description of the Consortium on Safe Labor is provided elsewhere.10, 11 Institutional Review Boards of all participating institutions approved the initial data collection. Institutional Review Board approval at the authors’ institution was active for all research using the database. All women with a twin gestation who delivered at ≥ 24 0/7 weeks gestation with a known height, prepregnancy weight and total gestational weight gain were included in the analysis. Exclusion criteria were intrauterine fetal demise of one or both twins and congenital anomalies.
The study cohort was stratified into three BMI groups: underweight/normal weight (BMI ≤ 25 kg/m2), overweight (BMI 25-30 kg/m2), and obese (BMI ≥ 30 kg/m2), as defined by their prepregnancy BMI. Each group was analyzed according to the weight gain during the pregnancy. Gestational weight gain was defined as the labor admission weight minus prepregnancy weight. Gestational weight gain was divided by the gestational age in weeks at delivery, to obtain the amount of weight gain per week. The IOM guidelines for weight gain for twin gestations were divided by 37 to obtain weight gain per week for each of the referent groups. For underweight/normal weight women, this was 0.46-0.68 kg/week (1-1.4 lbs/week). For overweight women, this was 0.38-0.62 kg/week (0.84-1.35 lbs/week). For obese women, this was 0.3-0.51 kg/week (0.68-1.13 lbs/week). The study groups are shown in Table 1.
Table 1.
Description of Study Groups, defined by gestational weight gain per week.
| Prepregnancy BMI | Weight gain below IOM Guidelines | IOM Guidelines for Weight Gain | Weight gain above IOM Guidelines |
|---|---|---|---|
| Underweight/Normal Weight | <0.46 kg/wk | 0.46-0.68 kg/wk | >0.68 kg/wk |
| Overweight | <0.38 kg/wk | 0.38-0.62 kg/wk | >0.62 kg/wk |
| Obese | <0.3 kg/wk | 0.3-0.51 kg/wk | >0.51 kg/wk |
The Institute of Medicine recommends 17-25 kg, 14-23 kg, and 11-19 kg weight gain for underweight/normal, overweight, and obese women with twins, respectively. These values were divided by 37 weeks to calculate the recommended weight gain per week (third column) along with low (second column) and high (fourth column) gestational weight gain for each prepregnancy body mass index category (first column).
Demographic information and maternal characteristics included age, race, education, chronic hypertension, pregestational diabetes, prepregnancy weight, prepregnancy BMI, gravidity, nulliparity, a prior preterm delivery, prior Cesarean delivery, drug use, alcohol use and smoking. The primary or neonatal outcomes were gestational age at delivery, birth weights of the larger and smaller twins, one or both twins categorized as small for gestational age, both twins < 2500 g, both twins < 1500 g and NICU admission. Secondary outcomes (maternal) were gestational hypertension, preeclampsia and gestational diabetes.
Means and standard deviations described the continuous variables with normal distribution whereas median and interquartile ranges described skewed data. All analyses were stratified by BMI. Categorical variables were presented as count and percentages. Demographics and maternal characteristics among the gestational weight gain categories were compared using ANOVA tests for continuous variables and Chi-square or Fisher exact tests for categorical variables. In a univariate analysis, the neonatal and maternal outcomes were compared between each of the weight gain categories defined above with either ANOVA, Chi-square, or Fisher exact tests. A p-value <0.05 was considered statistically significant. A multivariate analysis was then performed for each of the outcomes to control for confounding variables, including maternal age, parity, race and chronic hypertension. In these analyses, the referent group was the current IOM guidelines for weight gain in twin gestations: 0.46-0.68 kg/week for underweight/normal weight women, 0.38-0.62 kg/week for overweight women and 0.3-0.51 kg/week for obese women. For the categorical outcomes, logistic regression was used and adjusted odds ratios (aOR) with 95% CI were reported. For birth weight and gestational age at delivery, general linear regression was used and adjusted parameter estimates and p-values were reported. A p-value < 0.05 was considered significant. SAS software (version 9.2; SAS Institute Inc., Cary, NC) was used for all analyses.
Results
A total of 2654 women fit the inclusion criteria for the study. These women were stratified by prepregnancy BMI, 1497 in the underweight/normal weight group, 606 in the overweight group and 551 in the obese group. Demographics and maternal characteristics were compared in each BMI group by gestational weight gain (Table 2). In the underweight/normal weight group, only race, prepregnancy BMI, percentage of nulliparas, chronic hypertension, pregestational diabetes and drug use were significantly different. In the overweight group, age, chronic hypertension, pregestational diabetes and drug use were significantly different. In the obese group, chronic hypertension, pregestational diabetes and drug use were significantly different. There was no difference in smoking or alcohol use among the weight gain group, in any of the prepregnancy BMI categories.
Table 2.
Demographic information and maternal characteristics for each gestational weight gain group, stratified by prepregnancy BMI. Group 2 is the IOM provisional guidelines for gestational weight gain for each prepregnancy BMI group
| Underweight/Normal Weight | ||||
|---|---|---|---|---|
| Weight gain below IOM Guidelines N = 593 | IOM Guidelines for weight gain N = 671 | Weight gain above IOM Guidelines N = 233 | p | |
| Age (years) | 29.8 ± 6.6 | 30.6 ± 6.5 | 29.9 ± 6.9 | 0.08 |
| Nulliparas | 48.7% (289) | 54.6% (366) | 64.8% (151) | <0.01 |
| Chronic hypertension | 2.4% (14) | 2.5% (17) | 3.4% (8) | <0.01 |
| Pregestational diabetes | 1.5% (9) | 2.2% (15) | 1.3% (3) | <0.01 |
| Overweight | ||||
|---|---|---|---|---|
| Weight gain below IOM Guidelines N = 222 | IOM Guidelines for weight gain N = 247 | Weight gain above IOM Guidelines N= 137 | ||
| Age (years) | 28.7 ± 6.8 | 31.0 ± 6.9 | 30.0 ± 5.9 | <0.01 |
| Nulliparas | 30.6% (68) | 38.5% (95) | 40.2% (55) | 0.1 |
| Chronic hypertension | 1.8% (4) | 4.9% (12) | 6.6% (9) | <0.01 |
| Pregestational diabetes | 1.8% (4) | 1.6% (4) | 2.9% (4) | <0.01 |
| Obese | ||||
|---|---|---|---|---|
| Weight gain below IOM Guidelines N = 225 | IOM Guidelines for Weight Gain N = 179 | Weight gain above IOM Guidelines N = 147 | ||
| Age (years) | 28.7 ± 5.8 | 29.0 ± 5.6 | 29.1 ± 6.3 | 0.8 |
| Nulliparas | 25.3% (57) | 24.0% (43) | 33.3% (49) | 0.1 |
| Chronic hypertension | 5.3 % (12) | 10.6% (19) | 15.7% (23) | <0.01 |
| Pregestational diabetes | 4.4% (10) | 6.7% (12) | 7.5% (11) | <0.01 |
BMI= body mass index
Data presented as % (n) or mean ± standard deviation
In the underweight/normal weight group, the larger and smaller birthweights increased with increasing gestational weight gain. The percentage of twins born SGA decreased with increasing gestational weight gain. Birthweight < 2500 g decreased from 57.2% in those with gestational weight gain below the IOM guidelines to 36.2% in those with gestational weight gain above the IOM guidelines, p < 0.01. Birthweight < 1500 g decreased from 10.6% in those with a gestational weight gain below the IOM guidelines to 4.3% in those with gestational weight gain above the IOM guidelines. There was an increase in preeclampsia and gestational hypertension in those with gestational weight gain above the IOM guidelines However, there was no difference in gestational diabetes in those with gestational weight gain above the IOM guidelines, p=0.2 (Table 3).
Table 3.
Univariate analysis of perinatal outcomes for underweight/normal weight prepregnancy BMI group
| Underweight/Normal Weight | ||||
|---|---|---|---|---|
| Weight gain below IOM Guidelines N = 593 | IOM Guidelines for weight gain N = 671 | Weight gain above IOM Guidelines N = 233 | p | |
| Birthweight (g) | ||||
| Larger | 2433 ± 536 | 2610 ± 530 | 2718 ± 521 | <0.01 |
| Smaller | 2103 ± 578 | 2253 ± 523 | 2359 ± 498 | < 0.01 |
| Gestational age at delivery (weeks) | 35.1 ± 3.2 | 35.5 ± 2.7 | 35.7 ± 2.5 | <0.01 |
| Any twin NICU admission | 48.2% (286) | 42.0% (282) | 43.5% (101) | 0.09 |
| Any twin SGA | 16.2% (96) | 11.8% (79) | 9.1% (21) | <0.01 |
| Both twins <1500 g | 10.6% (61) | 5.2% (35) | 4.3% (10) | <0.01 |
| Both twins <2500 g | 57.2% (340) | 44.1% (296) | 36.2% (84) | <0.01 |
| Preeclampsia | 6.6% (39) | 8.9% (60) | 19.4% (45) | <0.01 |
| Gestational hypertension | 2.0% (12) | 4.2% (28) | 6.9% (16) | 0.01 |
| Gestational diabetes | 2.4 % (14) | 2.8% (19) | 1.3% (3) | 0.2 |
NICU = Neonatal intensive care unit
SGA = Small for gestational age
BMI body mass index
Data presented as % (n) or mean ± standard deviation
In the overweight group, the larger and smaller birthweights increased with increasing gestational weight gain, p < 0.01. Birthweights < 2500 g decreased from 53.6% in those with gestational weight gain less than the IOM guidelines to 35.8% in those with gestational weight gain above the IOM guidelines, p < 0.01. Preeclampsia increased with increasing gestational weight gain, p < 0.01. Gestational diabetes was less in the gestational weight gain above the IOM guidelines, 1.5%, compared to 6.5% with gestational weight gain at the IOM guidelines, p=0.04. There was no difference in other neonatal or maternal outcomes (Table 4).
Table 4.
Univariate analysis of perinatal outcomes for overweight prepregnancy BMI group
| Overweight | ||||
|---|---|---|---|---|
| Weight gain below IOM Guidelines N = 222 | IOM Guidelines for weight gain N = 247 | Weight gain above IOM Guidelines N= 137 | p | |
| Birthweight (g) | ||||
| Larger | 2452 ± 593 | 2586 ± 562 | 2696 ± 446 | <0.01 |
| Smaller | 2144 ± 607 | 2276 ± 601 | 2408 ± 473 | <0.01 |
| Gestational age at delivery (weeks) | 34.9 ± 3.3 | 35.5 ± 2.9 | 35.9 ± 2.2 | < 0.01 |
| Any twin NICU admission | 49.1% (109) | 45.3% (112) | 46.0% (63) | 0.7 |
| Any twin SGA | 13.1% (29) | 12.2% (30) | 6.6% (9) | 0.1 |
| Both twins <1500 g | 9.0% (20) | 8.1% (20) | 4.4% (6) | 0.3 |
| Both twins <2500 g | 53.6% (119) | 40.1% (99) | 35.8% (49) | <0.01 |
| Preeclampsia | 5.9% (13) | 10.5% (26) | 16.1% (22) | <0.01 |
| Gestational hypertension | 2.7% (6) | 3.2% (8) | 5.8% (8) | 0.6 |
| Gestational diabetes | 5.4% (12) | 6.5% (16) | 1.5% (2) | 0.04 |
NICU = Neonatal intensive care unit
SGA = Small for gestational age
BMI body mass index
Data presented as % (n) or mean ± standard deviation
In the obese group, the larger and smaller birthweights increased with increasing gestational weight gain, p < 0.01. Birthweights < 2500 g decreased from 60.4% in those with gestational weight gain less than the IOM guidelines to 34.3% in those with gestational weight gain above the IOM guidelines, p < 0.01. Preeclampsia, gestational hypertension and gestational diabetes all increased with increasing gestational weight gain, p < 0.01. There was no difference in the other neonatal or maternal outcomes (Table 5).
Table 5.
Univariate analysis of perinatal outcomes for obese prepregnancy BMI group
| Obese | ||||
|---|---|---|---|---|
| Weight gain below IOM Guidelines N = 225 | IOM Guidelines for weight gain N = 179 | Weight gain above IOM Guidelines N = 147 | p | |
| Birthweight (g) | ||||
| Larger | 2433 ± 536 | 2610 ± 530 | 2718 ± 521 | <0.01 |
| Smaller | 2155 ± 513 | 2257 ± 562 | 2370 ± 543 | <0.01 |
| Gestational age at delivery (weeks) | 35.0 ± 2.8 | 35.3 ± 3.0 | 35.6 ± 2.6 | 0.1 |
| NICU admit | 50.2% (113) | 44.4% (80) | 43.2% (63) | 0.3 |
| SGA | 10.7% (24) | 8.9% (16) | 8.2% (12) | 0.7 |
| Both twins <1500 g | 9.8% (22) | 6.1% (11) | 4.8% (7) | 0.2 |
| Both twins <2500 g | 60.4% (136) | 41.1% (74) | 34.3% (50) | <0.01 |
| Preeclampsia | 4.4% (10) | 15.0% (27) | 17.8% (26) | <0.01 |
| Gestational hypertension | 5.8% (13) | 6.1% (11) | 13.7% (20) | <0.01 |
| Gestational diabetes | 6.7% (15) | 6.1% (11) | 6.9% (10) | <0.01 |
NICU = Neonatal intensive care unit
SGA = Small for gestational age
BMI body mass index
Data presented as % (n) or mean ± standard deviation
In the multivariate analysis for neonatal outcomes (Table 6), all birth weights, except the smaller twin in the obese group, were significantly different than the IOM referent group. In this analysis, both twins < 2500 g significantly increased in those with gestational weight gain less than the IOM guidelines. Gestational weight gain above the IOM guidelines was protective against both twins < 2500 g in the underweight/normal weight group, aOR 0.7 (0.5-0.9). There was no difference in NICU admits with gestational weight gain above or below the IOM guidelines, in any of the groups.
Table 6.
Neonatal outcomes and IOM guidelines for GWG compared with linear (birthweight, adjusted p-values) and logistic regression (low birthweight, NICU admissions; aOR,95%CI), stratified by prepregnancy BMI
| Underweight/Normal Weight | |||
|---|---|---|---|
| Weight gain below IOM Guidelines N = 593 | IOM Guidelines for weight gain N = 671 | Weight gain above IOM Guidelines N = 233 | |
| Birthweight (g) | Referent | ||
| Larger | <0.01 | <0.01 | |
| Smaller | <0.01 | <0.01 | |
| Gestational age at delivery (weeks) | <0.01 | Referent | 0.5 |
| Both twins < 2500 g | 1.6 (1.3-2.1) | 1.0 | 0.7 (0.5-0.9) |
| Overweight | |||
|---|---|---|---|
| Weight gain below IOM Guidelines N = 222 | IOM Guidelines for weight gain N = 247 | Weight gain above IOM Guidelines N= 137 | |
| Birthweight (g) | Referent | ||
| Larger | <0.01 | 0.02 | |
| Smaller | <0.01 | 0.03 | |
| Gestational age at delivery (weeks) | 0.02 | Referent | 0.5 |
| Both twins < 2500 g | 1.8 (1.2-2.6) | 1.0 | 0.8 (0.5-1.3) |
| Obese | |||
|---|---|---|---|
| Weight gain below IOM Guidelines N = 225 | IOM Guidelines for weight gain N = 179 | Weight gain above IOM Guidelines N = 147 | |
| Birthweight (g) | Referent | ||
| Larger | <0.01 | <0.01 | |
| Smaller | 0.1 | <0.01 | |
| Gestational age at delivery (weeks) | 0.3 | Referent | 0.2 |
| Both twins < 2500 g | 2.2 (1.4-3.3) | 1.0 | 0.6 (0.4-1.0) |
NICU = Neonatal intensive care unit
Adjusted for maternal age, parity, race, prepregnancy BMI and chronic hypertension
In the multivariate analysis for maternal outcomes, preeclampsia increased at gestational weight gain above the IOM guidelines for underweight/normal weight women, aOR 2.4 (1.5-3.8). In the obese group, gestational weight gain less than IOM guidelines was protective against preeclampsia, aOR 0.2 (0.1-0.4). Gestational hypertension increased in the obese group with weight gain above the IOM guidelines, aOR 3.3 (1.4-7.7) (Table 7).
Table 7.
Logistic regression of maternal complications, stratified by prepregnancy BMI, compared to IOM recommended GWG
| Underweight/Normal | |||
|---|---|---|---|
| Weight Gain Below IOM Guidelines N = 593 | IOM Guidelines for Weight Gain N = 671 | Weight gain above IOM Guidelines N = 233 | |
| Preeclampsia | 0.6 (0.4-1.0) | 1.0 | 2.4 (1.5-3.8) |
| Gestational hypertension | 0.5 (0.2-1.0) | 1.0 | 1.7 (0.9-3.3) |
| Gestational diabetes | 0.8 (0.4-1.8) | 1.0 | 0.5 (0.1-1.8) |
| Overweight | |||
|---|---|---|---|
| Weight Gain Below IOM Guidelines N = 222 | IOM Guidelines for Weight Gain N = 247 | Weight gain above IOM Guidelines N= 137 | |
| Preeclampsia | 0.5 (0.2-1.0) | 1.0 | 1.6 (0.8-3.0) |
| Gestational hypertension | 0.8 (0.2-2.7) | 1.0 | 2.0 (0.7-6.1) |
| Gestational diabetes | 1.1 (0.5-2.4) | 1.0 | 0.2 (0.1-0.9) |
| Obese | |||
|---|---|---|---|
| Weight Gain Below IOM Guidelines N = 225 | IOM Guidelines for Weight Gain N = 179 | Weight gain above IOM Guidelines N = 147 | |
| Preeclampsia | 0.2 (0.1-0.4) | 1.0 | 1.1 (0.6-2.1) |
| Gestational hypertension | 1.3 (0.5-3.1) | 1.0 | 3.3 (1.4-7.7) |
| Gestational diabetes | 1.4 (0.6-3.1) | 1.0 | 1.0 (0.4-2.4) |
Adjusted for maternal age, parity, race, prepregnancy BMI and chronic hypertension
Comment
In this study, increasing mean birth weights of the larger and smaller twins were noted with increasing gestational weight gain. When gestational weight gain was less than the IOM guidelines, the odds of both neonates < 2500 g increased in all prepregnancy BMI groups. Significant improvements were seen especially in the underweight/normal weight group in all neonatal outcomes, except for NICU admissions, with gestational weight gain above the IOM guidelines. These improved neonatal outcomes were associated with minimal maternal risk, with increased odds of preeclampsia in the underweight/normal weight group with gestational weight gain above the IOM guidelines and increased odds of gestational hypertension in the obese group with gestational weight gain above the IOM guidelines.
Our study supports prior reports on neonatal outcomes and gestational weight gain. In 2007, Yeh reported on the ability to have a delivery at ≥ 36 weeks gestation and a birthweight ≥ 2500 g in twins.12 Their findings were similar to ours, showing that with a gestational weight gain ≥ 25 kg, there was a high likelihood of their composite outcome, as stated above. Except for obese women, the chance of a delivery at ≥ 36 weeks gestation and a birthweight ≥ 2500 g was significantly higher with higher gestational weight gain. 12 Luke at al also wanted to find which gestational weight gain categories were associated with ideal outcomes, defined in their study as birth weights between 2500 g and 2800 g. 13 With a gestational weight gain ≥ 35 lbs, there were significantly more women achieving the ideal outcome.13 The correlation between gestational weight gain and increasing birth weight were shown in another study from 2005. 14 In contrast to our study findings, a prospective cohort of twin gestations found no association with weight gain and neonatal outcomes. 15
Several other studies have reported on gestational weight gain in twins after the publication of the 2009 IOM guidelines. Similar to our findings, in term gestations (defined as ≥ 37 weeks gestation), the birth weights of both the larger and the smaller, as well as both twins with birth weights > 2500 g increased from 40% with weight gain less than the IOM guidelines to 80% with weight gain greater than the IOM guidelines.8 Likewise, another study found that with weight gain at or above the IOM guidelines, spontaneous preterm delivery occurred less frequently, mean birthweight was greater and both infants > 2500 g occurred more frequently.9 These findings remained significant even after stratification by prepregnancy BMI category. In another study, women with normal but not overweight or obese prepregnancy BMI's had larger birthweights and delivered at higher gestational ages with increasing gestational weight gain.7 More recently, one study showed that gestational weight gain at the IOM guidelines resulted in the best neonatal outcomes; and, interestingly, there was an increased risk of preterm delivery and low birth weights at all other gestational weight gain categories, whether higher or lower than the IOM guidelines.16
Our study is unique because the participants were from different medical centers in the United States, representing a diverse patient population. In addition, our study included a large number of total participants. We chose to include all women at ≥ 24 weeks gestation, with correction of total gestational weight gain by the number of weeks of gestation, as previously defined in other studies.8,9,16 We chose to follow this similar and previously published approach of dividing total gestational weight gain by gestational weeks at delivery. With this approach, we were able to evaluate gestational weight gain both less than and above the IOM guidelines, and made comparisons to gestational weight gain at the IOM guidelines. In the database, the prepregnancy and admission weight were available, so only total gestational weight gain could be studied. We acknowledge that studies discussing gestational weight gain are difficult to interpret due to the myriad of ways to calculate gestational weight gain, since it is correlated with length of the pregnancy.17 The study of maternal outcomes such as preeclampsia and gestational diabetes has been criticized if a total gestational weight gain is used instead of a weight gain at the time of the diagnosis of the outcome (reverse causality).1 However, other investigators have described an association between total gestational weight gain and preeclampsia and gestational diabetes.18-21 As such, we opted to report these outcomes in a secondary analysis as this information is overall limited for twin gestations.
Our study assessed the current IOM guidelines on gestational weight gain in twin gestations. Our study showed no increase in maternal adverse events with gestational weight gain above the IOM guidelines, with the exception of an increase in preeclampsia in the underweight/normal weight group with gestational weight gain above the IOM guidelines. We also showed a possible protective effect against preeclampsia with gestational weight gain less than the IOM guidelines in the obese group. Both of these, however, need to be confirmed with a greater sample size. While the optimal weight gain for twins has not yet been clearly defined, our study suggests that gestational weight gain above the IOM guidelines for twin gestations improves birth weight, especially for underweight and normal weight women, but needs further investigation in a larger prospective cohort.
Acknowledgements
The data included in this paper were obtained from the Consortium on Safe Labor, which was supported by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, through Contract No. HHSN267200603425C. Institutions involved in the Consortium include, in alphabetical order: Baystate Medical Center, Springfield, MA; Cedars-Sinai Medical Center Burnes Allen Research Center, Los Angeles, CA; Christiana Care Health System, Newark, DE; Georgetown University Hospital , MedStar Health, Washington, DC; Indiana University Clarian Health, Indianapolis, IN; Intermountain Healthcare and the University of Utah, Salt Lake City, Utah; Maimonides Medical Center, Brooklyn, NY; MetroHealth Medical Center, Cleveland, OH.; Summa Health System, Akron City Hospital, Akron, OH; The EMMES Corporation, Rockville MD (Data Coordinating Center); University of Illinois at Chicago, Chicago, IL; University of Miami, Miami, FL; and University of Texas Health Science Center at Houston, Houston, Texas. The named authors alone are responsible for the views expressed in this manuscript, which does not necessarily represent the decisions or the stated policy of the NICHD.
This study was supported by (1) the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health (NIH) (author M.A.K), through a contract (Contract No. HHSN267200603425C), (2) Grant Number K23 HD076010 from the NICHD and NIH (author MAK), and (3) the University of Illinois at Chicago (UIC) Center for Clinical and Translational Science (CCTS), Award Number UL1RR029879 from the National Center for Research Resources.
Footnotes
The authors report no conflict of interest.
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