Durie, Danielle E. MD, MPH; Thornburg, Loralei L. MD; Glantz, J. Christopher MD, MPH
In 2009, the Institute of Medicine revised their gestational weight gain recommendations, making several notable changes.1,2 The World Health Organization body mass index (BMI, calculated as weight (kg)/[height (m)]2]) classifications were adopted: underweight, less than 18.5; normal weight, 18.5–24.9; overweight, 25.0–29.9; and obese, 30 or greater.2 The previous recommendations of 28–40 lb for underweight, 25–35 lb for normal weight, and 15–25 lb for overweight women were maintained, and a lower minimal weight gain and a defined upper limit for obese women of 11–20 lb was added.2 Specific recommendations were not made for the individual obesity classes. The revisions also included recommended rates of second- and third-trimester weight gain in pounds per week: underweight, 1–1.3 lb; normal weight 0.8–1 lb; overweight, 0.5–0.7 lb; and obese, 0.4–0.6 lb.2
Numerous studies using the 1990 Institute of Medicine recommendations have shown an association between excessive gestational weight gain and pregnancy complications such as preterm birth, cesarean delivery, failed labor induction, large-for-gestational-age neonates, low Apgar scores, and neonatal hypoglycemia.3–12 Several reports also found that women with suboptimal weight gain have an increased risk of preterm birth and small-for-gestational-age neonates.8,12–17
More recent evidence suggests that obese women with excessive weight gain represent the highest risk group for adverse pregnancy outcomes.3,4,7,8,11,14,18–20 Several reports using the 2009 recommendations have shown excessive gestational weight gain to be associated with increased risk of large-for-gestational age neonates and weight loss to be associated with small-for-gestational-age neonates.21–25 The data are mixed regarding the association between suboptimal gestational weight gain and small-for-gestational-age neonates.21–25 No studies to date have used the recommended rates of second- and third-trimester weight gain provided by the Institute of Medicine to assess pregnancy outcomes.
The objective of this study was to estimate the effect of second- and third-trimester rate of gestational weight gain on maternal and neonatal outcomes in each prepregnancy BMI class using the revised Institute of Medicine recommendations.
MATERIALS AND METHODS
We performed a retrospective cohort study of all women who delivered singleton live births within the Finger Lakes Region of New York between January 2004 and December 2008. Women were identified using the Finger Lakes Region Perinatal Data System, a subset of the New York State electronic birth certificate database maintained by the New York State Department of Health.26 The Finger Lakes Region is comprised of nine counties with 13 hospitals that provide labor and delivery services, yielding approximately 14,500 births annually. The study was approved as a review of existing data by the University of Rochester Research Subjects Review Board.
Women were included if they delivered a singleton liveborn neonate at 20 weeks of gestation or greater. Prepregnancy BMI was calculated using the patient-reported prepregnancy weight and height as documented on the birth certificate. Women with missing prepregnancy weight, height, or delivery weight were excluded. Additional exclusions were women with recorded extremes of BMI (less than 15 or more than 90) or gestational weight gain or loss (more than 150-lb gain or more than 100-lb weight loss) as a result of biologic implausibility. Women were grouped into six BMI categories using the World Health Organization and National Institutes of Health classifications: underweight, less than 18.5; normal weight, 18.5–24.9; overweight, 25.0–29.9; obese class I, 30.0–34.9; obese class II, 35.0–39.9; and obese class III, 40 or greater.2,27
We chose to use the rate of second- and third-trimester weight gain because weight gain is not linear throughout the entire pregnancy, starting slower in the first trimester and becoming more uniform in the second and third trimesters.13 The use of rate of weight gain also allows for the inclusion of women with preterm delivery.28 The revised Institute of Medicine gestational weight gain recommendations include ranges for rate of second- and third-trimester weight gain (in pounds per week): underweight, 1–1.3; normal weight, 0.8–1; overweight, 0.5–0.7; and obese, 0.4–0.6.2
The rate of estimated second- and third-trimester weight gain was calculated as: [(gestational weight gain−estimated first trimester weight gain)/(gestational age−13 weeks)].2 Total gestational weight gain was calculated by subtracting the prepregnancy weight from the delivery weight, which we defined as the weight at the time of admission or as the last weight measured before delivery. Using the same references as in the Institute of Medicine report, we estimated the first-trimester weight based on prepregnancy BMI as 5 lb for underweight women, 4 lb for normal-weight women, and 2 lb for overweight and obese women, recognizing that there may be considerable variation across women in these values.2,6,15,29,30 Gestational age at delivery was determined by the best obstetric estimate of gestational age as recorded by the delivering physician. The first trimester was defined as the first 13 weeks of pregnancy.
Outcomes were assessed according to second- and third-trimester rate of weight gain. Within each BMI class, women were designated as being less than, within, or above the recommended rate of second- and third-trimester weight gain. Because there have been mixed reports about the effect of net weight loss on pregnancy outcomes, we divided the group of women with less than the recommended rates of weight gain into two groups: women who had a net weight loss (referred to as “weight loss”) and those who had zero or positive net weight gain (referred to as “less than recommended”).
The primary maternal outcome was cesarean delivery and primary neonatal outcomes were small-for-gestational age and large-for-gestational-age neonates. Cesarean delivery included both primary and repeat procedures. Birth weight percentiles were determined using the Finger Lakes Region birth weight curves, which are based on the local birth record registry within the Finger Lakes delivery region. Small for gestational age was defined as birth weight less than the tenth percentile and large for gestational age as greater than the 90th percentile for gestational age at delivery.
Secondary outcomes included induction of labor, gestational diabetes mellitus (GDM), and neonatal intensive care unit (NICU) admission. Induction of labor was defined as the use of mechanical cervical dilators, prostaglandins, or oxytocin for labor induction. Gestational diabetes mellitus was defined as diabetes diagnosed during the pregnancy according to an abnormal 100-g, 3-hour glucose tolerance test using the criteria from the National Diabetes Data Group.31 Although there is an established relationship between pre-eclampsia and weight gain, we elected to exclude it as an outcome because we cannot determine whether weight gain is the cause of or the result of pre-eclampsia.3,14
Additional data were available on maternal demographic characteristics and perinatal factors that may influence study outcomes, including maternal race and ethnicity, level of maternal education, tobacco use, nulliparity, chronic hypertension, and pre-existing diabetes. Maternal race and ethnicity was designated as white, African American, Hispanic, Asian, and other. Maternal education was self-reported as less than 12 years, 12 years, and greater than 12 years. Tobacco use included any tobacco use during pregnancy regardless of duration or amount. Nulliparity was defined as no prior births greater than 20 weeks of gestation. Chronic hypertension included pre-existing hypertension or hypertension diagnosed before 20 weeks of gestation. Pre-existing diabetes included only women diagnosed before pregnancy.
Differences in demographic and gestational weight gain characteristics were assessed across the six BMI groups using chi-squared analysis for categorical variables and analysis of variance for continuous variables. As a result of multiple comparisons in the analysis, statistical significance was set at P<.01. Backward logistic regression analysis was performed within each BMI class to determine the effect of the rate of second- and third-trimester weight gain on pregnancy outcomes. The regression analysis was performed for each rate of weight gain group: net weight loss, less than recommended, and greater than recommended using the recommended rate of gain as the reference group. The regression analysis controlled for maternal age, race (white as the reference), nulliparity, tobacco use, chronic hypertension, pregestational diabetes, and education level less than 12 years. Additionally, we controlled for prior cesarean delivery when analyzing the outcome of cesarean delivery. Only variables that remained statistically significant were retained in the regression analysis. Statistical analysis was performed using PASW Statistics 18.
There were 82,300 live births in the Finger Lakes Region during the study period with 73,977 women meeting inclusion criteria. We excluded 8,323 women as a result of extreme values of BMI or weight gain or loss (0.1%), gestational age under 20 weeks (0.3%), multiple gestation (38%), and missing height, weight data, or both (61%). Women who were excluded were more likely to be older, white, and deliver at earlier gestational age and less likely to use tobacco than included women. Within the cohort, 4% of the women were underweight, 48% normal weight, 24% overweight, and 24% obese (13% class I, 6% class II, and 5% class III).
Table 1 shows statistically significant differences among the BMI groups for maternal age, race, gestational age at delivery, and education level. Increasing BMI was associated with an increased incidence of chronic hypertension and pre-existing diabetes.
Table 2 shows the gestational weight gain variables for each BMI group. The mean net weight gain and second- and third-trimester rates of weight gain decreased with increasing BMI. Within the cohort, 62% of women had estimated second- and third-trimester rates of weight gain above the Institute of Medicine recommendations, although 21% were within and 17% less than the recommended rates. Net weight loss occurred in 1.5% of the entire cohort. The percentage of patients with weight loss increased with increasing BMI. Within the BMI groups, the highest proportion of excessive second- and third-trimester rates of gain were seen in overweight and obese class I women and the lowest in the underweight women.
The results from the logistic regression analysis for outcomes associated with net weight loss are shown in Table 3. The odds of a small-for-gestational-age neonate were increased only in those women with class I obesity with weight loss. Normal-weight women with weight loss had increased odds of their neonates requiring admission to the NICU. Table 4 shows the adjusted odds ratios for outcomes associated with less-than-recommended second- and third-trimester rates of weight gain. Underweight, normal weight, and overweight women had an increased odds of small-for-gestational-age neonates; however, this was not seen on obese women. Normal-weight women with suboptimal weight gain had increased odds of GDM and neonates with NICU admissions as well as decreased odds of large-for-gestational-age neonates and labor induction. Among women with class III obesity, no differences in maternal or neonatal outcomes were seen regardless of whether there was net weight gain or loss.
Table 5 shows the adjusted odds ratios for outcomes associated with greater-than-recommended second- and third-trimester estimated rates of weight gain. The odds of cesarean delivery were increased and the odds of small-for-gestational-age neonates decreased in normal weight, overweight, and obese class I and II women. In all BMI groups, excessive rates of weight gain were associated with increased odds of large-for-gestational-age neonates. Normal-weight women with excessive rates of weight gain also had increased odds of labor induction and neonates with NICU admissions as well as decreased odds of GDM.
In the wake of the ongoing obesity epidemic and the revised Institute of Medicine gestational weight gain recommendations, it is important to re-examine the effect of obesity and gestational weight gain on perinatal outcomes. The findings from this study show that maternal and neonatal outcomes are affected by the rate of second- and third-trimester weight gain. We found that for all but the most obese BMI groups, second- and third-trimester rates less than recommended by the Institute of Medicine were associated with increased odds of small-for-gestational-age neonates, although in contrast, a corresponding decreased risk of large-for-gestational-age neonates was not seen.
The association we found between suboptimal rates of weight gain and small-for-gestational-age neonates in the lower BMI groups is consistent with other studies.5,8,9,11,14 In underweight, normal-weight, and overweight women, the odds of having a small-for-gestational-age neonate was increased if the woman had a less-than-recommended weight gain during the second and third trimesters. However, in those same groups, net weight loss during pregnancy was not associated with small-for-gestational-age neonates. This is questionably plausible, and associations with net weight loss likely were not seen as a result of the small number of women in each of these BMI groups with a net weight loss.
The odds of small-for-gestational-age neonates were increased in women with class I obesity and weight loss, but not in women with class II or III obesity. Additionally, suboptimal rates of gain were not associated with small for gestational age in obese women. Our results suggest that low rates of weight gain, even with weight loss, in the most obese women do not increase the odds of having a small-for-gestational-age neonate. These findings are supported by Kiel et al and Bianco et al who both found minimal risk of small for gestational age in morbidly obese women with less-than-recommended weight gain.3,14 Our findings differ from several of the more recent studies focusing on obese women, all of which found an increased risk of small for gestational age associated with weight loss as well as suboptimal weight gain.21–23,25 Our results are more similar to that of Hinkle and colleagues, who noted increased odds of small for gestational age in women with class I obesity and weight loss and in class II and III obese women with excessive weight loss only (greater than 4.9 kg), but not in those with suboptimal weight gain.24
Our results showing increased odds of large-for-gestational-age neonates with excessive rates of weight gain are also consistent with a number of reports using net weight gain.3–5,7–10,14,21–25 Several studies using the 1990 Institute of Medicine recommendations5,9,14 and one with the revised guidelines21 found a positive association between high gestational weight gain and increased risk of cesarean delivery, similar to our results. Our only differences were seen in underweight women and women with class III obesity, who did not have an increased odds of cesarean delivery based on rate of weight gain. In the class III obesity group, this may be explained by the fact that 50% of women in this group underwent cesarean delivery, and rate of weight gain may only be one of many contributing factors.
Our study found increased odds of GDM in normal-weight women with suboptimal weight gain and decreased odds in women with excessive weight gain. Risk factors for diabetes are not routinely collected in our birth certificate database and therefore could not be controlled for. We postulate that women diagnosed with GDM may be more likely to adhere to a strict diet and increase exercise, ultimately resulting in lower weight gain. Conversely, women without a diagnosis of GDM may be less likely to follow a diet or exercise, which could lead to more weight gain.
A major strength of this study is the use of rate of weight gain instead of net weight gain to allow for the inclusion of women with preterm delivery. Many prior reports either exclude preterm patients from their cohort or include them adding potential bias to results. Additional strengths include the large sample size and the use of a database that uses trained coders and standardized definitions, which improves the generalizability of our results to other populations.
Our study has a number of limitations. The first is that this is a retrospective study, and accounting for all potential confounding variables is not possible. Many of the small but statistically significant differences in demographic characteristics are likely the result of the large sample size and have limited clinical significance. We did attempt to remedy this by using a P value of .01 and 99% confidence intervals as well as controlling for the variables that significantly differed among BMI groups. The potential for errors in birth certificate coding and underreporting of certain variables exists. Additionally, our analysis is restricted to the variables included on the birth certificate.
Additionally, our use of second- and third-trimester rates of weight gain involved an estimation of first-trimester weight gain using data from other populations and an assumed steady rate of gain throughout the second and third trimesters. These assumptions, although based on prior studies and varied by prepregnancy BMI, may introduce bias into the calculation; some error is certainly added for any given woman, because an average first-trimester weight gain was applied to all women. Because the use of the net rate of gestational weight gain, rather than second- and third-trimester rate of gain, may be the more appropriate method to assess associations between gestational weight gain and preterm birth, we did not include this outcome in our analysis.
In summary, we conclude that suboptimal and excessive rates of gestational weight gain are associated with adverse pregnancy outcomes in nonobese women. Most notably, nonobese women with suboptimal rates of weight gain have increased odds of small-for-gestational-age neonates, a finding not seen in the most obese women. Excessive rates of gestational weight gain are associated with large-for-gestational-age neonates irrespective of maternal BMI.
It is important to note that in the most obese women, however, limited weight gain and even net weight loss had no effect on small-for-gestational-age risk while potentially improving perinatal outcomes. This is a valuable tool in counseling patients. Our data suggest that it may be reasonable to further tailor the Institute of Medicine guidelines by lowering the weight gain recommendations in women with class II and III obesity in an attempt to optimize outcomes in this subclass of women with extreme morbid obesity. Based on the results from this study, the Institute of Medicine recommendations remain appropriate for all other women, including those with class I obesity.
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© 2011 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.