The epidemic of obesity is perhaps the most pressing health concern today. According to the latest data from the Centers for Disease Control and Prevention from 2012, 35.1% of women older than 20 years of age are obese (body mass index [BMI, calculated as weight (kg)/[height (m)]2] 30 or higher) and another 33.9% are overweight (BMI 25 or higher).1 Obesity is associated with reduced fertility, increased maternal and fetal risks during childbearing, and long-term health risks for women.2–7 Excess weight increases maternal risk of coronary artery disease, ischemic stroke, type 2 diabetes mellitus, osteoarthritis, gallbladder disease, hypertension, and some cancers including breast and colon.8
As a result, current recommendations include counseling women to achieve a normal weight status before becoming pregnant and to adhere to the Institute of Medicine guidelines for gestational weight gain. However, many women never lose the weight gained during a gestation, placing them at heightened risk for subsequent overweight and obesity in ensuing pregnancies and lifelong. Although addressing the obesity epidemic is critical for improving women's health, prior studies on risk factors for postpartum weight retention have shown conflicting results such as on the issue of parity9,10 and have not attempted to quantify the co-occurrence of potentially modifiable behaviors that are associated with postpartum weight retention. A clear understanding of the risk factors for postpartum weight retention is needed to start to combat the problem. The purpose of this study was to investigate the contribution of postpregnancy weight retention to obesity in young women at 1 year postpartum and to better understand modifiable factors that can lead to tailored interventions during and after pregnancy. In a national sample of predominantly low-income women drawn from the Eunice Kennedy Shriver National Institute of Child Health and Human Development Community Child Health Network, we evaluated risk factors for postpartum weight retention.
MATERIALS AND METHODS
Data were drawn from a national prospective cohort study conducted by the Eunice Kennedy Shriver National Institute of Child Health and Human Development Community Child Health Network. The Community Child Health Network is a group of community organizations and universities partnering to gain new insights into the contributors to the disparities in maternal health and child development. A 5-year observational study was completed according to principles of community-based participatory research to better understand multiple levels of maternal stress, resiliency, and allostatic load in the interconception period. The Community Child Health Network study enrollment sites include three urban (Baltimore, Los Angeles, and Washington, DC), one mixed urban and suburban (Lake County, IL), and one rural location (eastern North Carolina).
Most participants were enrolled in the hospitals after delivery. Eligible participants were women aged 18–40 years and with a live birth at 20 weeks of gestation or longer. Women were excluded if they were unable to give informed consent. Because one of the study objectives was to assess outcomes after future pregnancies, women who could not become pregnant or were unlikely to become pregnant were excluded. These exclusions were: surgical sterilization after the current delivery and having four or more children. The final sample comprised mostly low-income women who were African American, white, or Hispanic (both English speakers and Spanish speakers). Inclusion criteria for this secondary analysis additionally specified the availability of prepregnancy height and weight data; height at 6 months, 1 year, or both; and weight data at 1 year postpartum along with no subsequent pregnancy within 1 year postpartum.
Medical records were reviewed for the prepregnancy BMI and delivery information. Patients completed a brief baseline interview at enrollment followed by three extensive in-home interviews at 1 month, 6 months, and 12 months postpartum. Interviews were conducted in English and Spanish in accordance with respondent preference. Maternal height was measured by the interviewer at 6 months, 12 months, or both, whereas maternal weight was obtained by the interviewer at 6 months and 12 months postpartum.
Insurance status was coded as private or public insurance at the time of delivery as listed in the medical record. Respondents self-identified as African American, white, Hispanic, or other. The preferred language to conduct the interviews was used to delineate English-speaking and Spanish-speaking. Women provided their relationship status and whether the pregnancy had been planned on the initial screening interview while in the hospital. Education was defined by the highest level of schooling completed by the 1-month postpartum interview. Income level was characterized as poor at less than 200% Federal Poverty Level or nonpoor at more than 200% Federal Poverty Level based on self-report of household income and number of family members at the 1-month postpartum interview.
Weight was evaluated using the UC-321 Precision Person Health digital scale, which allowed a maximal weight of 350 lbs on both the 6-month and 12-month visits. Interviewers were instructed to place the scale on a hard surface, have the particiapnt take off her shoes, then have her stand on the scale and remain still until the measurement was displayed on the scale monitor. The weight then was recorded. For practical purposes, because the interviewers had to carry the scale to each patient's home, a scale that could measure weights higher than 350 lbs was not used. Height was measured in centimeters by study personnel on either the 6-month or 12-month visit or both. Interviewers determined the height by having women remove their footwear and stand on a flat, hard surface near a smooth vertical wall. The participant stood with heels against the wall with their feet and knees together. A wooden triangle was placed with the flat side against the wall and the perpendicular side along the head. A self-adhesive note was then placed at the right angle of the triangle. Height was measured with a rigid measuring tape from the bottom edge of the self-adhesive note straight to the floor and recorded. Body mass index was categorized according to the Institute of Medicine criteria of underweight less than 18.5, normal 18.5–24.9, overweight 25.0–29.9, and obese 30.0 or higher.
After the Institute of Medicine recommendations, women were considered to have gained an excessive amount of gestational weight if their prepregnancy BMI was normal and they gained more than 35 lbs, they were overweight before pregnancy and they gained more than 25 lbs, or they were obese before pregnancy and they gained more than 20 lbs.11
Women reported whether they were exercising at the 6-month interview. The amount of exercise was categorized based on the International Physical Activity Questionnaire with modifications to account for the postpartum state.12 Moderate exercise was defined as: 1) 3 or more days of vigorous activity of at least 20 minutes per day; 2) 5 or more days of moderate activity, walking of at least 30 minutes per day, or both; or 3) 5 or more days of any combination of walking, moderate or vigorous activities achieving a minimum total physical activity of at least 600 metabolic-equivalent minutes per week. Vigorous exercise was defined as 1) vigorous-intensity activity on at least 3 days achieving a minimum total physical activity of at least 1,500 metabolic-equivalent minutes per week; or 2) 7 or more days of any combination of walking, moderate-intensity, or vigorous-intensity activities achieving a minimum total physical activity of at least 3,000 metabolic-equivalent minutes per week.12 Moderate activity was defined as “activities that take moderate physical effort and make you breathe somewhat harder than normal.” Vigorous activity was described as “activities that take hard physical effort and make you breathe much harder than normal.” Any amount of exercise that did not meet the criteria for moderate or high was considered low.
At the 1-month and 6-month interviews, mothers were queried whether they ever breastfed the child and, if so, for how long. For those mothers who were still breastfeeding at the 6-month interview, duration was approximated as the age of the child at the date of the interview. Breastfeeding included both partial and exclusive. Women were asked about their smoking status and their average hours of nightly sleep at 6 months. Women also reported about their use of birth control and if they were working outside the home at the 1-year interview.
Bivariate analysis examined the relation between postpartum weight retention (less than 19 lbs and 20 lbs or more) and key variables. Differences in the distribution for each covariate by exposure status were assessed using Student's t tests, Wilcoxon rank sum, and χ2 analysis. Multivariable analysis was executed using nested linear regression. This method quantifies the additional contribution of the added variable group to the amount of variation in the dependent variable (postpartum weight retention 20 lbs or more) explained by the model. C-statistics were calculated. In successive models, we regressed 20 lbs or more of weight retention at 1 year postpartum on 1) nonmodifiable maternal factors such as age and race; 2) other socioeconomic demographics such as type of insurance, marital status, poverty level, weight gain during pregnancy, and prepregnancy BMI; and 3) potentially modifiable postpartum behaviors such as breastfeeding, working outside the home, and average of hours of nightly sleep. This cutoff was selected for the regression model because 20 lbs or more of weight retention constituted a more than 10% increase in weight for the majority of women in the study, which is clinically relevant and similar to the definition of weight retention used in other recent studies.13,14 The variables included in each model were based on the results of bivariate analysis. When variables were highly correlated (ie, insurance, poverty, income), only the variable with the largest variance explained was included in the multivariable model to avoid multicollinearity. For model comparison, we used the bootstrap method to calculate the 95% confidence interval of the C-statistic for each model. Throughout the data analysis, P values of <.05 were considered statistically significant. Analyses were conducted using SAS 9.3. Institutional review board approval was obtained at each site.
Of the 2,510 mothers enrolled in the Community Child Health Network, 774 had prepregnancy height and weight data, postpartum height and weight data, and did not become pregnant again within 1 year postpartum (Fig. 1). There were no statistically significant differences in the demographic characteristics of age, race, parity, or income level between the 774 women who had complete BMI data and no subsequent pregnancy within 1 year and the other 1,736 study participants. Data from the subset of 774 women were then used for the analysis.
Almost half the sample was African American with the remainder split between whites and Hispanics. Hispanics were further divided into mostly Spanish (64%) compared with English speakers (36%). Mean age was 26±5.8 years (range 18–42 years). Most women delivered a term neonate with a normal birth weight (more than 2,500 g or 5.5 lbs). Approximately one third had not planned the pregnancy and approximately one third were enrolled after delivering their first child (Table 1).
The mean prepregnancy weight was overweight: 161.5 lbs (range 82–345 lbs) with a BMI of 27.7 (range 15.8–54.6) (Table 2). More than half the sample had an unhealthy weight before pregnancy with 4.0% of women underweight, 39.8% normal, 26.4% overweight, and 29.8% obese. At 1 year postpartum, 2.7% were underweight, 29.8% were normal, 26.1% were overweight, and 41.3% were obese. The number of obese women significantly increased from 237 (29.8%) prepregnancy to 320 (41.3%) at 1 year postpartum (P<.001).
Strikingly, among the 39.8% women who had a normal BMI before pregnancy, 29.6% became overweight and 2% became obese by 1 year postpartum. Of the 26.5% women who were already overweight before pregnancy, 43.9% became obese by 1 year postpartum, whereas 96.9% of the women who were obese before the pregnancy remained obese at 1 year postpartum (P<.001).
Women gained a mean of 32 lbs during pregnancy. At 1 year postpartum, women retained a mean of 11.1 lbs (average 12 months postpartum BMI 29.4). Of the 774 women at 1 year postpartum, 579 (74.8%) were heavier than prepregnancy. Three hundred sixty-seven (47.4%) had retained 10 lbs or more and 187 (24.2%) had retained 20 lbs or more.
During the pregnancy, 416 (53.7%) of women gained more weight than the national recommendations.15 Thirty-seven percent of normal-weight women, 64.8% of overweight women, and 63% of obese women had excessive weight gain during the pregnancy. Women who gained more than the recommended amount during pregnancy were significantly more likely to be more than 20 lbs heavier than their prepregnancy weight (P<.001). Women with a normal prepregnancy weight who had excessive weight gain during pregnancy were also significantly more likely to retain more than 20 lbs of weight postpartum (P=.003).
Women who retained 20 lbs or more of postpartum weight were more likely to be African American, be younger, have a high school education, be poor, receive public aid, and to work outside the home. Women who retained more than 20 lbs were less likely to be in a relationship with the child's father, have planned the pregnancy, have breastfed, or exercised (Table 2). As expected, women who started the pregnancy with an overweight or obese BMI were at greater risk of retaining more than 20 lbs postpartum. However, approximately one in six women who started the pregnancy with a normal BMI also retained more than 20 lbs weight at 1 year postpartum.
We defined three sets of variables based on the strength of the bivariate relationships, face validity, or both, and then ordered these sets from least to most amenable to intervention. Then we conducted a series of nested linear regression models to predict postpartum weight retention in those who retained 20 lbs or more of postpartum weight (Table 3).
Model 1 identifies significant relations between maternal age and race. Older maternal age was protective against excessive postpartum weight retention (20 lbs or more) (odds ratio [OR] 0.96, 95% confidence interval [CI] 0.92–0.99). African American race (OR 2.21, 95% CI 1.31–3.71) was highly related to weight retention, whereas white race, Hispanic, and other race were not. Maternal age and race alone accounted for 65% of the variation in postpartum weight retention.
Older maternal age remains protective against excessive postpartum weight retention in model 2. In addition, model 2 indicates that each additional pound of weight gain during pregnancy above the Institute of Medicine recommendation increases the odds of excessive weight retention (20 lbs or more) by 8% at 1 year postpartum. Having a prepregnancy weight in the overweight BMI category was associated with an increase in the odds of postpartum weight retention of approximately 3.2-fold, whereas those who were obese before pregnancy had a 3.8-fold higher risk of retaining weight at 1 year postpartum. Of note, because poverty level was highly correlated with public insurance, it was dropped from this model. Neither public insurance nor single parenthood contributed to higher risk of excessive postpartum weight retention in our population. Overall, model 2 explains 80% of the variation in postpartum weight retention, a significant increase over model 1 of 15%.
Model 3, the final model, includes all model 1 and 2 variables and adds lifestyle behaviors. Moderate exercise was associated with a decrease in the odds of postpartum excessive weight retention (OR 0.16, 95% CI 0.40–0.93). Breastfeeding until 6 months was also a decrease in the risk of excessive postpartum weight retention (OR 0.46, 95% CI 0.24–0.87). Postpartum lifestyle behaviors, as measured in the Community Child Health Network, did not account for much additional variation in postpartum excessive weight retention with a small improvement only in the group that was obese prepregnancy. The final model explained 82% of the variation in postpartum weight retention, indicating a very strong explanatory model overall.
Our study confirms that pregnancy is a risk factor for women becoming overweight and obese, because many women do not lose the weight gained during pregnancy. Some other risk factors for postpartum weight retention in this predominantly low-income population included younger age, being African American, high prepregnancy BMI, excessive weight gain during pregnancy, not breastfeeding, and lack of exercise postpartum. Unlike prior studies, we did not find lack of sleep or lower parity to be associated with postpartum weight retention.15–17
There have been studies on the prevention of excessive pregnancy weight gain that have shown limited success.18,19 A review of four randomized controlled trials and five nonrandomized trials of strategies that focused on modifying gestational weight gain through physical activity and diet counseling showed that in all trials, the intervention groups had lower gestational weight gains than the control groups.20 Of particular note, given our findings that a significant number of women with a normal prepregnancy weight become overweight or obese by 1 year postpartum, interventions for preventing excessive pregnancy weight gain were more successful in normal-weight women than in women already overweight or obese.
The benefits of breastfeeding are well known for both mother and child. Women who breastfeed have decreased postpartum blood loss and more rapid involution of the uterus.21 They also have less postpartum depression.22 Our study also showed that women who met the American Academy of Pediatrics recommendation for 6 months of breastfeeding were significantly less likely to have retained the weight gained in pregnancy at 1 year postpartum. The substantial benefit of breastfeeding to maternal and child health should be communicated to women by health care providers during the pregnancy and reiterated postpartum.
One of the limitations of our study was the number of women who were excluded because a prepregnancy BMI was not available. However, we compared demographic characteristics between women with and without a prepregnancy BMI and found no statistically significant differences. Also this group of women was not selected randomly from the general population and they were oversampled for low income and preterm birth. Although there is no reason to believe these women differ from other postpartum women in other ways, this may make the study less generalizable. Some of the socially undesirable variables such as smoking were self-reported, which may be underreported. Although we show an salutary effect of breastfeeding and exercise only in the group that was obese before pregnancy, this limited effect may be an artifact of Community Child Health Network's measurement strategy in that we could not separate partial from exclusive breastfeeding, and physical activity was self-reported and may be overreported. Nonetheless, we are encouraged to see that small changes have benefits among the most overweight (who are most at risk). As in any study, there may be other unexplored variables, but our model was very strong for explaining excessive postpartum weight retention (20 lbs or more).
In our opinion, providers of health care for pregnant women need to know the Institute of Medicine recommendations, actively discuss weight gain during pregnancy with all patients—including those of normal weight—monitor weight gain, encourage daily exercise, and support nutrition counseling. Postpartum, health care providers should emphasize the target of breastfeeding for 6 months and adopting a regular program of moderate exercise, particularly in women at high risk for postpartum weight retention: pregnancy at a younger age, African American race, high prepregnancy BMI, and excessive weight gain during pregnancy. Because obesity and its health consequences are critical national health issues, creative, synergistic interventions are urgently needed.
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