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Viswanathan M, Siega-Riz AM, Moos MK, et al. Outcomes of Maternal Weight Gain. Rockville (MD): Agency for Healthcare Research and Quality (US); 2008 May. (Evidence Reports/Technology Assessments, No. 168.)

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Outcomes of Maternal Weight Gain.

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Executive Summary

Introduction

This systematic review of outcomes of gestational weight gain, often referred to as maternal weight gain, is motivated by several trends in perinatal health that are of great public health concern. Women are increasingly gaining weight during pregnancy beyond the thresholds set forth by the Institute of Medicine (IOM); increases are pronounced among overweight and obese women; obesity levels among women of childbearing ages are rising dramatically; and pregnancy complications associated with excess gestational weight gain such as large-for-gestational-age babies and cesarean delivery have increased in prevalence. These trends point to the need for assessment of the guidelines to address optimal weight gain for all women during pregnancy.

The RTI International-University of North Carolina at Chapel Hill Evidence-based Practice Center (RTI-UNC EPC) conducted a systematic review of the literature to review the evidence on influence of gestational weight gain on various outcomes. We systematically assessed the evidence for five key questions (KQs):

KQ 1. What is the evidence that either total weight gain or rate of weight gain during pregnancy is associated with (1) birth outcomes, (2) infant health outcomes, and (3) maternal health outcomes? Does any evidence suggest that either total weight gain or rate of weight gain is a causal factor in infant or maternal health outcomes?

KQ 2. What are the confounders and effect modifiers for the association between gestational weight gain (overall and patterns) and birth outcomes? Based on the findings in KQ 1, do these confounders and effect modifiers themselves contribute to antepartum or postpartum complications or to longer-term maternal and fetal complications, including development of adult obesity?

KQ 3. What is the evidence that weight gains above or below thresholds defined in the 1990 IOM body mass index (BMI) guidelines or weight loss in pregnancy contribute to antepartum or postpartum complications or longer-term maternal and fetal complications? How do these relationships vary by sociodemographic characteristics (i.e., race and age)?

KQ 4. What are the harms or benefits of offering the same weight gain recommendations to all pregnant women, irrespective of age and body weight considerations (e.g., pregravid weight, actual body weight at a particular time point, or optimal body weight)?

KQ 5. What are the anthropometric tools for determining adiposity and their appropriateness for the pregnancy state? What are the risks and benefits of measuring adiposity for (1) clinical management of weight gain during pregnancy and (2) evaluation of the relationship between weight gain and outcomes of pregnancy?

Methods

We searched MEDLINE®, Cochrane Collaboration resources, Cumulative Index to Nursing & Allied Health Literature, and Embase. We dually reviewed each study against a priori inclusion/exclusion criteria. For included articles, a primary reviewer abstracted data directly into evidence tables; a second senior reviewer confirmed accuracy. We included 150 studies in English, published from 1990 through October 2007. We excluded studies with low sample size (cases series < 100 and cohorts < 40) or failure to control for pregravid weight. We rated individual studies for quality, based on the assessment of nine domains of quality: background, sample selection, specification of exposure, specification of outcome, soundness of information, followup, analysis comparability, analysis of outcome, and interpretation. In assessing the strength of evidence for each outcome as strong, moderate, weak, or absent, we incorporated the quality of the studies in addition to consistency and volume of the evidence.

Results

KQ 1 and KQ 3: Outcomes of Gestational Weight Gain

KQ 1 asks about outcomes of gestational weight gain for infants and for mothers; more than 30 outcomes were specified as being of interest. KQ 1 also asks what evidence exists to demonstrate causality. Nearly all the studies in this review are observational studies; therefore, generally this evidence base cannot demonstrate a causal link between gestational weight gain and outcomes. The analysis of outcomes related to weight gains in relationship to IOM recommendations (KQ 3) classified outcomes into six categories as with KQ 1: maternal antepartum outcomes, maternal intrapartum outcomes, birth outcomes (neonatal outcomes at the time of birth), infant outcomes (<1 year), child outcomes (≥ 1 year), and short- and long-term maternal outcomes. To enable synthesis and help identify gaps in the evidence, we combined discussion of the findings for KQ1 and KQ3 and focus below on outcomes for which the evidence is either strong or moderate.

Maternal intrapartum outcomes. We examined the literature for 11 maternal and intrapartum outcomes. For one of these—cesarean delivery—the evidence was of moderate strength. For all other outcomes, evidence was weak.

Cesarean delivery. Of the 21 studies identified in KQ 1 (14 fair, 7 poor), all but 4 showed some degree of association between higher weight gain and cesarean delivery; the evidence was moderate. The association appeared to be stronger among overweight and obese women.

Nine studies (8 fair, 1 poor) examined the association between gestational weight gain classified by the IOM guidelines and cesarean delivery (KQ 3). These studies suggest moderate evidence for increased risk of cesarean for weight gain above IOM recommendations for underweight and normal weight women, and weak inconsistent evidence for obese or morbidly obese women.

Overall, the majority of studies suggested an association between weight gain and cesarean delivery. Our findings of a higher risk of cesarean for overweight and obese women, coupled with the lack of significance of weight gain above IOM recommendations among obese or morbidly obese women, suggests that underlying health risks (such as increased risks of abnormal glucose tolerance) associated with high pregravid weight are likely confounders in the relationship between gestational weight gain and cesarean delivery.

Birth outcomes. The knowledge base about the association between gestational weight gain and birth outcomes is, on the whole, stronger than the knowledge base for any other set of outcomes with moderate to strong evidence for low gestational weight gain and preterm birth, low birthweight, and small-for-gestational-age (SGA) birthweights, and strong evidence for the association between high weight gain and high birthweight, macrosomia, and large-for-gestational-age (LGA).

Preterm birth. Strong evidence from 12 studies (2 good, 7 fair, 3 poor) suggests that both low and high weight gains result in an increased risk of premature birth (KQ 1). Despite little consistency in terms of adjustment for covariates, definition of preterm birth (with or without premature rupture of membranes), and the methods used to define and categorize gestational weight gain, eight of nine studies reported at least one significant association between low gestational weight gain and preterm birth; four of five studies focused on high gestational weight gain (as defined in each study) reported at least one significant association between gestational weight gain and preterm birth.

Among the studies that examined preterm birth using categories of gestational weight gain (i.e., low, adequate, and high), seven of eight reported a significant increased risk of preterm birth with low gestational weight gain and four of five reported a significant increased risk with high gestational weight gain. In general, low rates of weight gain were ≤ 0.37 kg per week and high rates of weight gain were > 0.52 kg per week throughout gestation.

Four studies, all of fair quality, reported on the association between rate of gestational weight gain according to the IOM guidelines and preterm birth (KQ 3). Despite inconsistencies in the definitions and timing of rate of weight gain calculations, the four studies are consistent in showing increased risks of preterm birth for underweight and normal-weight women who have a low rate of gain.

Overall, the majority of studies found a consistent effect of low gestational weight gain on preterm birth, and a less consistent effect of high gestational weight gain on preterm birth. The association for low gestational weight gain holds whether total weight gain or rate of weight gain is used as the relevant exposure of interest.

Birthweight. Evidence from 25 studies (4 good, 12 fair, 9 poor) provided strong evidence that gestational weight gain is associated with infant birthweight (KQ 1). This relationship held true for various measures of gestational weight gain. Evidence from seven studies reported that birthweight increased between 16.7 and 22.6 g for every 1 kg increase in weight gain. Two studies reported values by BMI status, suggesting that the effect of increased gestational weight gain on infant birthweight was more pronounced at lower BMI levels. Three studies examined the effect of weight gain by trimester on infant birthweight. All three studies were consistent in demonstrating the least effect of gestational weight gain in the third trimester. Two of three studies that used similar definitions of trimester found that a 1-kg increase in gestational weight gain during the first trimester was associated with 18–31 g increases in birthweight, whereas during the second trimester, such gains were associated with increases of 26–32.8 g, and increases of 7–17 g during the third trimester.

10 articles (1 good, 8 fair, and 1 poor) from nine databases provide strong evidence that weight gain below IOM recommendations is associated with lower birthweights (KQ 3). Seven studies found an association between high weight gains and higher birthweights, particularly for underweight and normal-weight women.

Overall, we found strong evidence in support of an association between gestational weight gain and birthweight. Low gestational weight gain is associated with lower birthweights across all pregravid weight status groups whereas high gestational weight gain resulting in higher birthweight appears to be limited to underweight and normal-weight women.

Low birthweight (LBW). Thirteen studies (one good, nine fair, three poor) provided strong evidence that low weight gain increases the risks of LBW (KQ 1).

Ten studies published in twelve articles (two good, seven fair, and three poor) provided strong, consistent evidence of an association between weight gain below the IOM guidelines and LBW for only underweight and normal-weight women (KQ 3).

Overall we found strong evidence for an association between low gestational weight gain and low birthweight. The evidence appears to be stronger among women of underweight and normal-weight pregravid status than among overweight and obese women.

Macrosomia. Eleven of 12 studies (1 good, 9 fair, 1 poor) provided strong evidence that high gestational weight gain is associated with greater risks of macrosomia (KQ 1). The relationship between high gestational weight gain and macrosomia held despite variations in definition of macrosomia (> 4,500 g or > 4,000 g). Generally, the highest weight gains were associated with the highest risks of macrosomia.

Seven studies examined the association between gestational weight gain categorized according to the IOM and macrosomia defined as either > 4,000 g or > 4,500 g (2 good, 1 poor, the remaining fair) (KQ 3). These studies suggest moderate evidence for the association between weight gains above the IOM recommendations and macrosomia for overweight and obese women.

Overall, moderate to strong evidence suggests that high weight gains are associated with macrosomia.

Large-for-gestational-age (LGA). Fourteen studies (one good, eight fair, five poor) with varying definitions of weight gain and LGA were consistent in demonstrating an association between high gestational weight gain and LGA; we graded the evidence for this association as strong (KQ 1). This association held whether LGA was defined as birthweight greater than the 90th percentile, or as birthweight more than two standard deviations above the mean. Whether BMI modifies this relationship is unclear.

Eight articles examined gestational weight gain according to the IOM and LGA (2 good, 5 fair, 1 poor) (KQ 3). These studies provided strong evidence that high weight gains are associated with an increased risk of LGA infants. Weight gains below IOM guidelines, by contrast, were protective against LGA in only four studies (moderate evidence).

Overall, we found strong evidence of the association between high gestational weight gain and LGA despite differences in the definition of LGA.

Small-for-gestational-age (SGA). Twenty publications (1 good, 12 fair, and 7 poor) provided strong evidence that women in the lowest weight gain categories had higher percentages of SGA infants and were at increased risk of delivering an SGA infant, despite differences across studies in the definition of weight gain and SGA (KQ 1). In general, the risk of SGA among women with low weight gain decreased as BMI increased.

Ten articles (three good, one poor, and the rest fair) examined the association between gestational weight gain categorized according to the IOM guidelines on SGA (KQ 3). The evidence is strong that SGA is associated with weight gains below the IOM guidelines. Weight gains above the IOM were associated with a lower risk of SGA in four out of six studies suggesting a moderate degree of evidence.

Overall, we found strong evidence for an association between low gestational weight gain and the risk of having an SGA infant.

Maternal short- and long-term outcomes. The literature covered eight maternal outcomes. Of these, the evidence is moderate for only intermediate-term (3 months to 3 years postpartum) postpartum weight retention (PPWR) for both KQ 1 and KQ 3. In addition, we found moderate strength of evidence to support the association between gestational weight gain and breastfeeding initiation, short-term PPWR (≤ 11 weeks postpartum) and long-term PPWR (> 3 years postpartum). The evidence for all other outcomes (breastfeeding duration, fat accrual, interpregnancy weight retention, and premenpausal breast cancer) was weak (except for lactation, for which no literature was available at all).

Breastfeeding initiation. Three studies, published in four articles, all fair, examined the association of weight gain in relation to the IOM guidelines and breastfeeding (KQ 3) and provide moderate evidence of an association between weight gains below the IOM guidelines and lower likelihood of breastfeeding initiation.

Short-term PPWR (≤ 11 weeks). Four studies (all fair) provide moderate evidence that weight gains exceeding IOM guidelines (KQ 3) were associated with PPWR measured at or before 11 weeks postpartum.

Intermediate PPWR (3 months to 3 years). Five studies (one good, three fair, and one poor) provide moderate evidence for an association between gestational weight gain and intermediate PPWR (KQ 1).

Six studies (five fair and one poor) examined the association between weight gain categorized according to the IOM and intermediate-term PPWR (KQ3). They consistently indicated that women who gained above the IOM recommendations retained more weight than those who gained within the guidelines (moderate strength of evidence). They provided only weak evidence about any association when weight gains were below IOM guidelines.

Overall, we found moderate evidence for an association between high gestational weight gains and intermediate PPWR.

Long-term postpartum weight retention (>3 years). Three fair studies provided moderate evidence of an association between high gestational weight gain, defined according to the IOM (KQ 3), and weight retention later in life, but the magnitude of weight retained was small.

Other outcomes. We found no moderate or strong evidence for maternal antepartum outcomes (hyperemesis, abnormal glucose metabolism, hypertensive disorders, gallstones, and maternal discomforts of pregnancy), infant outcomes (neonatal hypoglycemia, neonatal distress, hyperbilirubinemia, neonatal hospitalization, other infant morbidity, infant BMI, and other infant growth characteristics), or childhood health outcomes (childhood obesity and childhood hospitalization).

KQ 2: Confounders and Effect Modifiers of Gestational Weight Gain

KQ 2 asks about the confounders and effect modifiers relevant for examining any associations between gestational weight gain (overall and patterns) and birth outcomes; it also asks about the extent to which these confounders and effect modifiers themselves contribute to outcomes. As reported in our results and discussion for KQ 1, the types of confounders and effect modifiers vary considerably by the type of outcome being considered. Little consistency exists within the body of evidence for each outcome on which confounders are to be included, and even less consistency exists on their definition.

Given the large variations in the overall body of evidence on the confounders and effect modifiers, our discussion of KQ 2 is limited to outcomes with moderate or strong evidence of association with gestational weight gain. These are preterm birth, mode of delivery, birthweight, low birthweight, macrosomia, LGA infants, SGA infants, and intermediate-term PPWR.

Because age, race and ethnicity, and pregravid weight status are key considerations in the 1990 IOM weight gain recommendations, we further limit our discussion of KQ 2 to the consideration of these key variables. We additionally consider parity. The included studies defined these four variables in highly variable fashion. Finally, because KQ 2 asks about the independent association between confounders and effect modifiers, we considered results only from multivariate analyses for confounders and effect modifiers that included gestational weight gain as a predictor of the outcome. These studies together provide strong evidence of the independent association of pregravid weight status on outcomes, moderate evidence on age and parity, and weak evidence, largely because of insufficient data, on the effect of race.

KQ 4: Benefits and Harms of Gestational Weight Gain Recommendations

Research is inadequate to permit objective assessment of harms and benefits of providing all women, irrespective of age, race or ethnicity, or pregravid BMI, with the same recommendation for weight gain in pregnancy. The majority of the studies present evidence suggesting that one recommendation for all women would be disadvantageous, but the findings are not consistent and do not fully explore harms. Most studies limited their analyses to short-term outcomes related to the pregnancy, the intrapartum, and the neonatal period. A full examination of harms would require long-term, rigorously designed studies to determine unexpected consequences of specific recommendations; we found no such investigations.

KQ 5: Anthropometrics of Gestational Weight Gain

Our review required that included studies estimated adiposity using body weight or BMI. Ten studies collected data using other anthropometric measurements and incorporated them into varying body composition equations or models to estimate body fat.

These measurements included bioelectrical impedance analysis, dual energy X-ray absorptiometry, body circumferences, total body water, total body potassium, magnetic resonance imaging, and underwater weighing. Collectively, these studies do not provide sufficient evidence to judge whether alternate methods of weight measurement are more informative or predictive of infant and maternal outcomes than standard body weight (including BMI) and height measurements.

Discussion

We found strong evidence to support the association between gestational weight gains and preterm birth, birthweight, macrosomia, LGA, SGA, low birthweight, and preterm birth. Moderate evidence supported the association between gestational weight gain and breastfeeding initiation, mode of delivery, and PPWR.

As reported in our results and discussion for KQ 1, the types of confounders and effect modifiers vary considerably by the type of outcome being considered. Little consistency exists within the body of evidence for each outcome on which confounders are to be included, and even less consistency exists on their definition. The studies reviewed provide strong evidence for the independent association of pregravid weight status and outcomes, moderate evidence for age and parity, and weak evidence for race and ethnicity.

Existing research is inadequate to permit objective assessments of the range of harms and benefits of providing all women, irrespective of age, race or ethnicity, or pregravid BMI, with the same recommendation for weight gain in pregnancy.

Clear clinical recommendations based on this systematic review will be challenging to formulate because of major shortcomings in the body of research investigating gestational weight gain and pregnancy outcomes. The research is almost all observational; it lacks uniformity of definitions, methodologies, and analyses. To understand fully the impact of gestational weight gain on the short- and long-term outcomes for women and their offspring will require that researchers use consistent definitions of gestational weight gain and the outcomes, describe the criteria used to assess confounding in their analysis, use statistical methods that allow for the evaluation of more than one outcome at a time, make improvements in study design to allow better collection of weight and weight gain data, and follow women and infants for longer periods.

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