GDM: Abnormal Glucose Tolerance During Pregnancy (2008)

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

This study examined the relationship of glucose intolerance during pregnancy to birth weight among black and white participants of the Pregnancy, Infection, and Nutrition (PIN) Study.

 

 

Inclusion Criteria:
  • The PIN study recruited 2,898 women from August 1, 1995 - May 31, 2000.
  • PIN study protocol were reviewed and approved by the Institutional Review Board of the School of Medicine at University of North Carolina (UNC) at Chapel Hill and Wake Medical Center.
  • Women recruited between 24 and 29 weeks of pregnancy were eligible to participate in the study if they were having singleton pregnancies.
  • >16 years of age
  • Spoke English
  • Access to phone
  • Planned to continue their care and deliver at one of the study sites

 

All women were classified into 1 of 3 groups:

  • IGT was defined as one abnormal value from the OGTT
  • NGT was defined as women with a normal or high GCT but no high values on the OGTT
  • GDM defined by two or more abnormal values on the OGTT.

 

Exclusion Criteria:
  • 31 with preexisting diabetes
  • 314 who had no screening glucose data
  • 63 who had a high screen without an oral glucose tolerance test and thus could not be classified
  • 23 with missing birth outcome data
  • 178 for whom BMI could not be imputed
  • 140 from racial groups other than white or black.
  • In addition, 94 women had two pregnancies in the cohort; their second pregnancies were excluded.
Description of Study Protocol:

Recruitment method was not detailed

Design Prospective Cohort

Blinding used:  not applicable

Intervention  Women with GDM received both diet and or insulin treatment at both clinics and women with IGT were not treated at either clinic.

Statistical Analysis  

  • Initial data preparation was performed using the SAS6.12 statistical package with analysis conducted using STATA release 6.0.
  • Descriptive statistics of the main outcomes and exposures were calculated.
  • X2 analysis was used to test for differences in categorical variables, and a Student’s t test or one-way ANOVA was used to test for differences in continuous variables.
  • Logistic regression was used to examine the association between glucose tolerance status and the outcomes of macrosomia and LGA.
  • Odds ratios and 95% CIs were calculated.
  • Multiple linear regression was used to examine the association between glucose tolerance status and birth weight.
  • Interaction in the linear regression model was assessed by evaluating significance at P< 0.1. Because interaction between glucose status and race

 

Data Collection Summary:

Timing of Measurements :

Women were contacted by phone within 2 weeks of recruitment for completion of a questionnaire. Information was gathered on current and pregravid health behaviors, including physical activity and sociodemographic characteristics.

Dependent Variables

Diagnostic Criteria: Glucose tolerance information was obtained from both hospital computer data bases and the medical charts.

  • Glucose status was determined using a 2-step approach
  • The initial screening test measured the plasma glucose concentration 1-h after 50-g glucose challenge test (GCT). This random screen did not require that women be in the fasting state. The cut-points used to indicate the need for a full 3-h 100-g OGTT was different for each clinic because of a disagreements about which cut-point represents risk. The UNC site used a value of > 140mg/dL; the second site (Wake Medical Center) used > 130 mg/dL.
  • The OGTT conducted in the fasting state with glucose analysis performed at fasting and at 1, 2, and 3 h after the glucose loads. The tests were performed on serum samples using a glucose oxidase method. Carpenter and Coustan cut-points of 95 mg/dL for fasting, 180 for 1-h, 155 for 2-h, and 135 for 3-h values were used to define abnormal values.

Newborn growth outcomes: obtained from labor and delivery logs on all women.

  • Birth weight (in grams) as a continuous variable
  • Macrosomia defined as birth weight > 4, 00 g. Macrosomia was examined to provide an evaluation of high birth weight irrespective of gestational age and to make comparisons with other previously published studies.
  • Large for Gestational Age (LGA) was defined as sex-, race-, and parity-specific birth weight for gestational age above the 90th percentile of U.S. population fetal growth curves. LGA was examined to provide information about gestational age-specific growth rates.

Independent Variables

  • Race - self-identified by women during the interviewer-administered telephone questionnaire or from the medical charts if missing
  • Pregravida BMI (in kilograms divided by meters squared) was classified following guidelines established by the Institute of Medicine (IOM), which are <19.8 (underweight), 19.8-26 (normal), 26-30 (overweight), and > 30 (obese). Height measured clinic visits and prepregnancy weight based on women’s recall were used for calculations. If recalled prepregnancy weight was not available then the first measured weight between 1 and 15 weeks’ gestation was used, with a correction factor for the week of measurement.
  • Gestational age was defined as completed weeks, based on last menstrual period and/or the earliest ultrasound assessment.

Information from the telephone interview was used to construct additional variables that were assessed as potential confounders

  • Family income was ascertained and used to calculate poverty index according to U.S. Bureau of Census 1996 poverty guidelines.
  • Cigarette smoking habits during the first 6 months of the pregnancy
  • Regular or strenuous physical activity –participation 3 months before becoming pregnant
  • Marital status
  • Parity
  • mother’s age
  • education

Control Variables

 

Description of Actual Data Sample:

Initial N: The PIN study recruited 2,898 women from August 1995 to May 31,2000. Of the 2,898 successfully recruited women

Attrition:  2,055 were retained for this analysis.

Age: See Table 1

Ethnicity: Whites, Blacks

Other relevant demographics:

Anthropometrics

Location: The Department of Nutrition, School of Public Health, University of North Carolina, Chapel Hill, North Carolina.

 

Summary of Results:

Significant differences existed between whites and blacks on most demographic variables (Table 1).

Table 1- Selected characteristics of the study population stratified by race 

-

White

Black

n

1,190

865

Maternal age (years)

1,19027.4 + 6.4

86524.1+ 5.4*

Maternal height (inches)

1,9065.4 + 2.5

86565 + 2.8

BMI categories

-

-

Under

226(19)

121(14)*

Normal

623(52)

363(42)

Over

127(11)

116(13)

Obese

214(18)

265(31)

Marital status

-

-

Single

273(23)

605(70)*

Married

822(69)

196(23)

Other

94(8)

63(7)

Maternal education (years)

1,18814.1 + 3.2

86412.5 + 2.1*

Poverty index (n=1,778)

-

-

Low <185%

445(41)

540(77)*

Medium 185-350 %

229(21)

127(18)

High > 350%

403(37)

34(5)

Parity

1,1870.76+ 0.93

8620.98 + 1.2*

Smoker (n=1,885)

335(30)

145(19)*

Participated in physical activity

352(31)

110(14)*

Gestational age at delivery(weeks)

1,19038.9 + 2.1

86538.6 + 2.3¶

1-h 50-g GTC (mg/dl)

1,185110 + 27

864102 + 25*

Data are n and means + SD or n (%). * Indicates significant difference between black and white women, P< 0.001 ; ¶ indicates significant difference between black and white women, P < 0.01.

Black women with IGT had higher rates of both LGA and macrosomia compared with white women with IGT (Table 2). 

Table 2-Prevalence of glucose tolerance status and selected birth outcomes by glucose status and race. 

-

<<<

White

(n=1,190)

>>

>>>

Black (n=865)

<<<<<<

-

Normal

IGT

GDM

Normal

IGT

GDM

n

1,080

40

70

820

13

32

Birth weight (g)

3385 + 620

3463+ 499

3453+ 676

3184+ 609

3,800 + 726

3,271 + 614*

Macrosomia (>4,000 g)  

14.8(160)

10.0(4)

20.0(14)

7.0(57)

38.5(5)

6.1(2)¶

LGA*

11.5 (117)

13.2(5)

19.1(13)

11.6(94)

53.9(7)

18.9 (6)¶

Data are n and means + SD or n (%). * Indicates a sample size reduction due to missing LGA values(whiten=1,126, black n =859) ¶ indicates significant difference between glucose tolerance status among race-specific groups, P<0.0001.

Black women with IGT were at greater risk of both macrosomia and LGA compared with black women with NGT(Table 3).

Table 3- Crude and adjusted OR and 95% CIs for macrosomia and LGA by glucose tolerance status

-

White>>

(n=1,190)

Black>>

(n=865)

-

Crude RR

(95% CI)

Adjusted OR¶

(95% CI)

Crude RR

(95% CI)

Adjusted OR¶

(95% CI)

IGT*

-

-

-

-

Macrosomia

0.6 (0.2-1.8)

0.6 (0.2-1.7)

8.4 (2.7-26)

5.7 (1.6-20)

LGA

1.2 (0.4-3.0)

1.1 (0.4-2.9)

8.9 (2.9-27)

7.7 (2.3-26)

GDM*

-

-

-

 

Macrosomia

1.4 (0.8-2.6)

1.1 (0.6-2.1)

0.9 (0.2-3.8)

0.7 (0.2-3.3)

LGA

1.8(0.9-3.4)

1.4 (0.7-2.7)

1.8 (0.7-4.4)

1.6 (0.6-4.2)

*Referent group: normal glucose tolerance; ¶ models adjusted for prepregnancy, BMI, mother’s age, and height

Other Findings

 

Author Conclusion:

This study suggested that, independent of maternal prepregnancy weight, there may be significant increased risk of macrosomia among black IGT women but not among white IGT women. Further investigations into factors that may contribute to the observed results are needed.

Limitations:

  • Sample size for the analysis was small.
  • It is possible that more proximate etiological factors, which were not measured in the study, may account for the relationships observed.
  • Residual confounding associated with the measured variables may be influencing the relationship.
  • The small number of IGT cases among black women (n=13), limited the precision of the estimated risk.
Funding Source:
Reviewer Comments:

Analytical longitudinal surveys refer to what epidemiologists term prospective or cohort studies. A Cohort Study is a study in which patients who presently have a certain condition and/or receive a particular treatment are followed over time and compared with another group who are not affected by the condition under investigation. Studies of this kind provide a better opportunity than one time cross sectional studies to examine whether certain behaviors do in fact lead to (or cause) the disease.                

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

 

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) Yes
 
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? Yes
  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? Yes
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  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? Yes
  4.1. Were follow-up methods described and the same for all groups? N/A
  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%.) N/A
  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? N/A
  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.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  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? Yes
  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? Yes
  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? Yes
  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? Yes
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