Obesity has reached epidemic proportions in the United States, with 31% of individuals in the United States considered to be obese (body mass index [BMI] ≥30, WHO standard).1 Deleterious effects of obesity may be especially pronounced in pregnant women, with obese mothers being at higher risk for primary cesarean section, labor induction, hypertension, preeclampsia, stillbirth, and gestational diabetes mellitus.2 Gestational diabetes (commonly defined as glucose intolerance first detected during pregnancy) is known to increase the likelihood of subsequent type II diabetes in the mother, with the fetus also susceptible to increased mortality and morbidity.3 Some studies suggest that children of mothers with gestational diabetes might be at an increased risk for impaired motor function, minor neurologic deficits, and shortened attention span.4,5
Recent cohort studies have estimated the incidence of gestational diabetes among obese women to range from 6% to 11%.6–8 An increase in prepregnancy weight between pregnancies has been associated with an increased risk for recurrent gestational diabetes,9 and women with a prepregnancy weight of at least 190 lbs have been shown to be 70% more likely to have a recurrence of gestational diabetes than their lighter counterparts.10
Although obesity is an established risk factor for gestational diabetes,11 the effect of weight reduction between pregnancies has not been explored. Previous studies have suggested that weight gain between pregnancies increases the risk of recurrent gestational diabetes,12 but it is not known whether weight changes between pregnancies can affect the risk of incident gestational diabetes.
Our goal was to explore the extent to which weight change between pregnancies may modify the risk of the first known occurrence of gestational diabetes among obese women. We examined the risk of developing gestational diabetes among women who were obese at first pregnancy and lost or gained at least 10 lbs before the beginning of their subsequent pregnancy, relative to obese women with less weight change.
Study subjects were selected from the Washington State Longitudinal Births Database,13 which contains linked birth certificate information for all women with at least 2 births in Washington during 1984–1998. The Institutional Review Board of the Washington State Department of Health approved research use of these data and linkages performed in the conduct of this study. Information concerning maternal prepregnancy weight and pregnancy weight gain was added to the birth certificates in 1992. We selected subjects for this analysis from among women with at least 2 live births during the years 1992–1998. Each woman's first 2 births on record during this time were screened to identify subjects for the study cohort. These births were not necessarily a woman's first and second births ever, but were rather the first (index) and second (subsequent) births on record in Washington State during this time period. Eligible subjects weighed at least 200 lbs prepregnancy and did not have gestational diabetes or established diabetes (type I or II) at their index pregnancy. Women for whom this information was unavailable were excluded (n = 484). We identified 4102 women with at least 2 live births on record who were 200 lbs or more and nondiabetic at index pregnancy.
As a result of the lack of information on height, we defined obesity in terms of weight alone rather than BMI. Although BMI is more commonly used to define obesity, previous studies have found that maternal weight is a reasonably effective predictor of gestational diabetes and other outcomes among pregnant women.10,14,15 We identified 200 lbs as a weight near or above the cutoff for clinical obesity (BMI of 30 or more) for most female height categories. Based on a linkage of birth certificate and drivers’ license data, women's height information was available for some women with births during 1992–1998. Thus, we were able to validate our definition of obesity among a sample of 40,699 women with available BMI data who experienced at least 2 births. Using BMI ≥30 as the gold standard definition of obesity, the 200-lb cutpoint had a sensitivity of 53% and a specificity of more than 99%. We also evaluated ≥180 lbs (sensitivity = 82%, specificity = 97%) and ≥250 lbs (sensitivity = 9%, specificity = 100%) as potential cutpoints; the 200-lb cutpoint offered excellent specificity while preserving enough power to detect an important reduction of risk.
Birth certificate data are recorded shortly after the time of delivery by the mother or her healthcare providers and usually are submitted electronically to the State Department of Health. The process of collecting and recording birth certificate information, however, may have varied by hospital, as well as by individual, and may include information obtained directly from the mother, from a review of the medical record, from the care providers, or from some combination of these.
Weight change was measured as the difference between prepregnancy weights for the index pregnancy and the subsequent pregnancy. The prepregnancy weight recorded on the birth certificate is generally the weight recorded at the first prenatal visit, but some weights may have been self-reported. Subjects were categorized into 3 groups of interpregnancy weight change: lost at least 10 lbs (n = 859), gained at least 10 lbs (n = 1914), and gained or lost less than 10 lbs (n = 1329). The outcome was gestational diabetes status at the subsequent pregnancy reported in a checkbox format on the birth certificate. Relative risks (RRs) and 95% confidence intervals (CIs) of gestational diabetes were obtained by conducting stratified analyses and calculating Mantel-Haenszel estimates, and by logistic regression. We evaluated as potential confounders the following maternal characteristics for which data were available from the birth certificates: maternal age (less than 35 years/35+ years), race (black, white, Asian, other), Hispanic ethnicity, education level (less than 12 years/12 years or more), marital status (married/not married), prenatal smoking status, chronic hypertension, preeclampsia, weight gain during pregnancy (0, 1–14, 15–24, 25–35, 36+ lbs), parity (0, 1, 2+ prior births), and interbirth interval (less than 18, 19–35, 36+ months). Hospital discharge data included type of insurance billed at hospital discharge (Medicaid, HMO/commercial, other); we used this as a proxy of socioeconomic status. Factors that altered the risk estimates by 10% or more were included in the adjusted relative risk estimates. Unless otherwise indicated, final risk estimates were adjusted for maternal age and weight gain during the index and subsequent pregnancies.
We also examined possible effect modification by index prepregnancy weight. Subjects were categorized into 2 groups: prepregnancy weight between 200 and 249 lbs (n = 3505), and prepregnancy weight at least 250 lbs (n = 597). The relative risk of gestational diabetes was examined within each of these groups with assessment of confounding similar to the main analysis.
Among the 4102 women in the study who were obese at their first pregnancy, 2511 (61%) gained weight between their index and subsequent pregnancies, and 1299 (32%) women lost weight. Among the women who gained weight, the average weight gain was 23 lbs; among women who lost weight, the average weight loss was 24 lbs. Information on diabetes status at both pregnancies was available for 1914 women who gained at least 10 lbs, 859 women who lost at least 10 lbs, and 1329 women who lost or gained less than 10 lbs between pregnancies. Subjects were fairly similar across categories of interpregnancy weight change with respect to age, race, ethnicity, marital status, medical insurance billed at discharge, and education level at subsequent pregnancy (Table 1).
Women who lost at least 10 lbs between pregnancies were slightly more likely to smoke during their subsequent pregnancy (21%) than women who gained at least 10 lbs (16%). Weight gain during pregnancy was inversely related to interpregnancy weight change. Among women who lost at least 10 lbs between pregnancies, 39% then gained more than 35 lbs during their subsequent pregnancy, whereas among women who gained at least 10 lbs between pregnancies, only 11% gained more than 35 lbs during their subsequent pregnancy. Additionally, women who gained at least 10 lbs between their index and subsequent pregnancy experienced greater weight gain during their index pregnancy than the other 2 groups (data not shown). Parity was also slightly associated with weight change; women who gained at least 10 lbs between pregnancies were more likely to have been nulliparous at the index pregnancy (58%) compared with women who lost at least 10 lbs (48%).
Women who lost at least 10 lbs between pregnancies had a relative risk (adjusted for maternal age at second pregnancy and weight gained during index and subsequent pregnancies) of 0.63 (95% CI = 0.38–1.02) of developing gestational diabetes during their subsequent pregnancy as compared with women whose weight changed less than 10 lbs between pregnancies (Table 2). The relative risk of diabetes at the subsequent pregnancy for women who gained weight between pregnancies was 1.47 (95% CI = 1.05–2.04) relative to those who gained or lost <10 lbs between pregnancies. Logistic regression was also performed to analyze the robustness of our weight change cutpoints. Each 1-lb increase in interpregnancy weight change was associated with an OR of 1.01 (95% CI = 1.00–1.02).
When subjects were stratified by weight before the index pregnancy, weight loss and weight gain were associated with gestational diabetes risk only among women who weighed between 200 and 249 lbs at index pregnancy (Table 3). Weight change was not associated with gestational diabetes risk in women who weighed at least 250 lbs before their index pregnancy.
Obesity is a modifiable risk factor with far-reaching consequences. The majority of women of childbearing age have periodic contact with the healthcare system, giving providers the opportunity to promote behavior changes that may result in weight loss. Our results suggest that obese women who lose at least 10 lbs between pregnancies have a nearly 40% decreased risk of gestational diabetes in a subsequent pregnancy relative to women without weight change. This may decrease the risk to themselves and their infants for several adverse outcomes related to gestational diabetes. Conversely, obese women who gain at least 10 lbs between pregnancies are at a substantially increased risk of gestational diabetes. Our findings are consistent with the literature on recurrence of gestational diabetes,12 and the observation that weight loss between pregnancies is associated with a lower risk of gestational diabetes supports our original hypothesis. We also observed that the association between interpregnancy weight change and gestational diabetes was modified by the mother's weight at index pregnancy. The impact of weight change on risk of gestational diabetes was only detectable among women weighing between 200 and 249 lbs.
Our findings should be considered in light of several limitations. Because height information was not available, we defined obesity in terms of weight alone rather than BMI. Although BMI is more commonly used to define obesity, maternal weight has been shown to be similarly effective in predicting disease risk in populations of pregnant women.10,14 Indeed, BMI itself serves only as a surrogate for body fat content, which is rarely available for analysis. Additionally, using weight to define obesity may be more relevant to women and their clinicians, because women and their physicians are more likely to know their weight than their BMI. The specificity estimate (greater than 99%) from our validity analysis indicates that the weight-based definition of obesity (as 200 lbs or more) led to the inclusion of very few, if any, nonobese women in our population. However, our exposure definition had limited sensitivity (53%) and failed to capture all of the obese women in the longitudinal birth certificate file. Also, as a result of the low numbers of nonwhites in our dataset, we were unable to examine possible effect modification by race.
The birth certificate data collection process was not designed to examine the association between interpregnancy weight change and gestational diabetes. However, we believe that the data were adequate to address our questions of interest. Although birth certificate data go through several quality checks, the potential for missing information and misclassification for several variables remains. Nondifferential misclassification may dilute the effect we are seeking to detect, or it may allow for residual confounding after stratification. Because maternal weight may have been self-reported by some women in our sample, and because underestimation of weight is common among overweight women,16 some truly obese women may have been excluded from the present study. It is likely that such underestimation did not lead to substantial exposure misclassification, because underestimation would probably be similar at each pregnancy. In addition, there is the possibility of incomplete ascertainment of gestational diabetes status on the birth certificate. Because obesity is a known risk factor for gestational diabetes, obese women may be subject to closer screening, and this could lead to a spurious or exaggerated association between interpregnancy weight change and gestational diabetes. However, because screening for diabetes was an established standard of prenatal care during the study time period, and the majority of subjects (97%) reportedly had prenatal care before their subsequent pregnancy, the inclusion of women with unidentified diabetes is unlikely. Also, we were unable to assess the class of gestational diabetes from the birth certificate.
Given the observational nature of this study, we must be cautious about causal inference. Modifiable risk factors for gestational diabetes include physical activity, dietary fat, and other lifestyle factors,17 which could confound the relationship between weight loss and gestational diabetes. In particular, mothers with recurrent gestational diabetes have higher fat intake,18 and weight gain between pregnancies could plausibly be associated with dietary fat. No consensus yet exists as to either the optimal weight-loss diet for obese women who want to reduce their risk of developing gestational diabetes19 or the efficacy of diets with this goal.20
Despite the established health risks associated with obesity, more than half of the obese, nondiabetic mothers in our cohort gained weight by the time of their subsequent pregnancy. To some extent this may be the result of expected weight gain associated with pregnancy and with advancing age.21 In addition, there is an established association between high prepregnancy weight or BMI and even higher postpartum weight or BMI.22,23 Because weight gained during pregnancy (which increases with age24) is not always lost after birth,25 it is not surprising that many of the women in our study gained weight between pregnancies. As obesity has become more prevalent among childbearing women, uniform pregnancy weight gain recommendations have been questioned.26 Recent guidelines emphasize the importance of setting pregnancy weight goals with prepregnancy weight in mind, rather than using a one-size-fits-all recommendation.27 For this reason, pregnant women should work with their physicians to establish a target weight based on individual factors with the evaluation of weight changes during both prenatal visits and routine checkups postpartum.
Our data provide encouraging evidence that weight loss, and even weight maintenance, can help reduce the health risks among obese women of childbearing age.
We thank Cathy Critchlow, Bill O'Brien, Lisa Godefroy Johnson, Susan Searles Nielsen, and Tiffany Harris for their assistance. We also thank the Washington State Department of Health for permission to use these data.
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