GDM: Prevention of GDM Diagnosis (2008)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To determine whether weight loss between pregnancies reduced the risk of gestational diabetes among obese women.

Inclusion Criteria:
  • Subjects were selected from the Washington State Longitudinal Births Data Base, which contained linked birth certificate information on 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 the linkages performed in the conduct of this study.
  • Information concerning maternal prepregnancy weight and pregnancy weight gain was added to the birth certificate in 1992.
  • Subjects for this analysis were selected from among women with at least 2 live births during the years 1992-1998.
  • The births were the first (index) and second (subsequent) births on record in Washington State during 1992-1998.
  • Eligible subjects weighed at least 200 lbs prepregnancy and did not have gestational diabetes or established diabetes (type 1 or II) at their index pregnancy.
Exclusion Criteria:
Exclusion criteria was not delineated.
Description of Study Protocol:

Recruitment : Study subjects were selected from the Washington State Longitudinal Births Database which contained linked birth certificate information for at least 2 births in Washinton during 1984-1998.

Design:  Population-based cohort study

Blinding used (if applicable):  not applicable 

Intervention (if applicable):  not applicable 

Statistical Analysis : Relative risk (RRs) and 95% confidence intervals (CIs) of gestational diabetes were obtained by conducting stratified analysis and calculating Mantel-Haenszel estimates, and by logistic regression.

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.

A second analysis examined the possible effect modification by an index prepregnancy weight. Subjects were catergorized into two 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.

 

Data Collection Summary:

Timing of Measurements: Birth certificate data are recorded shortly after the time of delivery by the mother or her health care provider and are usually submitted electronically to the State Department of Health.

Dependent Variables: gestational diabetes status at subsequent birth among obese women (reported in a check box format on the birth certificate).

Obesity defined as: 200 lbs as a weight near or above the cutoff of for clinical obesity (BMI of 30 or more). Weight ,alone, was the determining factor for obesity, as height information was not available. The study validated the BMI concept through the utilization of the  linkage of birth certificate and drivers'license data,womens' heights were available for some women during the targeted time period of 1992-1998.

Validation of definition of obesity among a sample of 40,699 women with available BMI who experienced at least 2 births. Using BMI>=30 as gold standard definition of obesity, the 200-lb cutpoint had a sensitivity of 53% and a specificity 99%. The 200 lb cut-point offered excellent specificity while preserving enough power to detect an important reduction of risk.

Subjects were categorized into 3 groups of interpregnancy weight change:

  • Lost or gained less than 10 lbs
  • Lost 10 lbs or more
  • Gained 10 lbs or more
  • Obesity was determined in terms of weight alone rather than BMI, due to a lack of information on height.

Independent Variables

  • Mother's age (< 35 years of age)
  • Race - Black, White, Asian, Other
  • Hispanic
  • < 12 years of education
  • Marital status
  • Insurance billed at hospital discharge (Medicaid, HMO/Commercial, other) 
  • Smoked prenatally
  • Weight gain during second pregnancy-weight changes were measured as the difference between for the index pregnancy and the subsequent pregnancy.
  • Interbirth interval (months)
  • No. of births before index pregnancy
  • Index prepregnancy weights (lbs.)  

Control Variables

 

Description of Actual Data Sample:

Initial N: 4120 women with at least 2 live births on record and who were 200 lbs or more and nondiabetic at index pregnancy.

Attrition (final N): 4120

Age: See Table I

Ethnicity: See Table I

Other relevant demographics:

Anthropometrics

Location: University of Washington, Seattle, Washington.

 

Summary of Results:

Subjects  were fairly similar across categories of interpregnancy weight change with respect to age, race, ethnicity, maritaal status, medical insurance billed at discharge, and educational level at subsequent pregnancy (Table I).

Table 1.  Characteristics of Obese Women by Categories of Weight Change Between Index and Subsequent Pregnancies*

 

  Weight >>> Change
  Decrease¶ Stable§ Increase£
Mother <35 years old

87

84

88

Race      
Black

3

2

4

White

86

86

85

Asian

2

2

2

Other

9

10

10

Hispanic

6

7

8

<12 years education

21

15

17

Married

78

84

81

Insurance billed at hospital discharge      
Medicaid

 38

 32

37 

HMO/Commercial

 53

 57

 52

Other

 9

 10

 11

Smoked prenatally

 21

 17

 16

Chronic hypertension

 2

 3

 4

Weight gain during 2nd pregnancy (lbs)      
1-14

 9

 21

 31

15-24

 20

 30

 29

25-35

 30

 27

 22

36+

 39

 17

 11

Lost<=0

 2

 5

 7

interbirth interval (months)      
<=18

 20

 22

 17

19-35

 55

 58

 54

36+

 25

 19

 30

No. of births before index pregnancy      
0

 48

 53

 58

1

 32

 27

 26

2+

 20

 20

 17

Index prepregancy weights (lbs.)      
200-249

 82

 85

 87

250+

 18

 15

 13

 * Characteristic are measured at the second pregnancy unless otherwise noted.

¶ >=10 lbs

§ < 10 lbs.

Women who had lost at least 10 lbs between pregnancies had a relative risk (adjusted for maternal age at second pregnancies and weight gain during index and subsequent pregnancies) of 0.63 (95% CI= 0.38-1.02) of developing gestational diabetes during their subsequent pregnancy as compared to women whose weight changed less than 10 lbs between pregnancies (Table 2).

Table 2. Risk of Gestational Diabetes During Subsequent Pregnancy Among Obese Women Who Lost or Gained at Least 10 lbs Between Pregnancies, Relative to Those Whose Weight Stayed Near-Constant

   

Gestational

Diabetes at

Subsequent Birth

 

n

NO.(%)

RR*(95%CI)

Adjusted RR¶

(95% CI)

Loss or gained less than 10 lbs £

 1329

 61(4.6)

 1.0

 1.0

 Lost 10 lbs or moremore

 859

 28(3.3)

 0.66(0.43-0.99)

 0.63(0.38-1.02)

Gained 10 lbs or more

1914

119(6.2)

1.3(0.98-1.7)

1.47(1.05-2.04)

*Unadjusted

¶ Adjusted for maternal age at subsequent pregnancy,weight gain during index pregnancy,and weight gain during subsequent pregnancy.

£Reference category 

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).

Table 3. Risk of Gestatioal diabetes During Subsequent Pregnancy Among Obese Women Who Lost or Gained at Least 10 lbs Between Pregnancies, Relative to Those Whose Weight Stayed Near-Constant, Stratified by Index Prepregnancy Weight

     Gestational  Diabetes at  Subsequent Birth 

 Index Prepregnancy Weight

 Weight Change

 n

 No.(%)

 RR*(95%CI)

 200-249 lbs

 Lost or gained less than 10lbs£

 1132

 50(4.4)

 1.0

 

 Lost 10 lbs or more

 702

 19(2.7)

 0.50(0.28-0.92)

 

 Gained 10 lbs or more

 1671

 102(6.1)

 1.56(1.08-2.24)

>=250 lbs

Lost or gained less than 10 lbs £

197

11(5.6)

1.0

  Lost 10 lbs or more

157

9(5.7)

1.18(0.42-3.29)

  Gained 10 lbs or more

243

17(7.6) 1.02(0.47-2.18)

*Adjusted for maternal age at subsequent pregnancy,weight gain during index pregnancy,and weight gain during subsequent pregnancy.                                                                         

£ Reference category.

 

Author Conclusion:

Even moderate changes in prepregnancy weight can apparently affect the risk of gestational diabetes among obese women.

The authors presented a caveat about the causal inference of study results. It was noted that no consensus yet exists as to either the optimal weight-loss diet for obese women who want to reduce their risk of developing gestational diabetes or the efficacy of diets with this goal.

Validation of definition of obesity among a sample of 40,699 women with available BMI who experienced at least 2 births. Using BMI>=30 as gold standard definition of obesity,the 200-lb cutpoint had a sensitivity of 53% and a specificity 99%. The 200 lb cut-point offered excellent specificity while preserving enough power to detect an important reduction of risk.

Limitations

  • Height information was not available, obesity was defined based on weight alone rather than BMI.
  • The limited specificity estimate (greater than 99%) from the validity analysis  indicated that  the weight based definition of obesity (as 200 lbs or more) led to the inclusion of very few, if any nonobese women in the population studied.
  • The limited sensitivity (53%) resulted in a failure to capture all of the obese woemn in the longitudinal birth certificate file.
  • As a result of the low numbers of nonwhites in the data set, researchers were unable to examine possible effect modification by race.
  • Although birth certificate data go through several quality checks, the potential for missing information and misclassification for several variables remained.
  • Because maternal weight may have been self-reported by some women in the sample, and because underestimation of weight is common in overweight women, some truly obese women may have have been excluded from the study.
  • There is the possibility of incomplete ascertainment of gestational diabetes on the birth certificate.
  • Researchers were unable to assess the class of gestational diabetes from the birth certificate. 
Funding Source:
Government: NHLBI
Reviewer Comments:

The limitations and critique of the study, as stated by the authors appear to be very appropriate. 

In population-based cohort studies, a sample of a defined population is selected for longitudinal assessment of exposure-outcome relations. Because of their typically high cost and logistic complexities, population-based cohort studies generally evaluate multiple hypotheses, some defined a priori as well as some suggested in the course of the study either based on interim analyses or on advances in the field, particularly when data are repeatedly collected on cohort members. Perhaps the most often cited justification for conducting a population-based study is its external validity—that is, the applicability of its results to a defined population.

Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) N/A
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? ???
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? No
3. Were study groups comparable? ???
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? ???
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? Yes
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? N/A
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? Yes
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? N/A
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? N/A
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? ???
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? Yes
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes