RESEARCH DESIGN AND METHODS—This was a cohort study of 14,448 GDM pregnancies delivered between 1995 and 2006. Postpartum screening was defined as performance of either an FPG or OGTT at least 6 weeks after delivery and within 1 year of delivery.

RESULTS—Between 1995 and 2006, the age- and race/ethnicity-adjusted proportion of women who were screened postpartum rose from 20.7% (95% CI 17.8–23.5) to 53.8% (51.3–56.3). Older age, Asian or Hispanic race/ethnicity, higher education, earlier GDM diagnosis, use of diabetes medications during pregnancy, and more provider contacts after delivery were independent predictors of postpartum screening. Obesity and higher parity were independently associated with lower screening performance. Among women who had postpartum screening, the age- and race/ethnicity-adjusted proportion of IFG did not change over time (24.2 [95% CI 20.0–27.8] in 1995–1997 to 24.3 [22.6–26.0] in 2004–2006), but the proportion of women with diabetes decreased from 6.1 (95% CI 4.2–8.1) in 1995–1997 to 3.3 (2.6–4.0) in 2004–2006. Among women who received an OGTT in 2006, 38% of the 204 women with either diabetes or pre-diabetes were identified only by the 2-h glucose measurements.

Performance rates of postpartum diabetes screening have been low (5–7), but screening performance may have changed recently. At present, only one population-based report has examined postpartum diabetes screening practices, and this report examined fasting plasma glucose (FPG) only (8). We used data from a GDM registry in a large prepaid group practice managed health care organization (the Kaiser Permanente Medical Care Program in Northern California [KPNC]) and examined 1) postpartum diabetes screening over time, 2) predictors of postpartum screening in a detailed electronic medical record, 3) trends in impaired fasting glucose (IFG) or diabetes detected with postpartum screening, and 4) the proportion of women with diabetes or pre-diabetes identified by the FPG screen versus the proportion of women with these abnormal glucose values identified by the 75-g oral glucose tolerance test (OGTT).

We used the KPNC GDM registry (11) to identify women with GDM who delivered between 1 January 1995 and 31 December 2006. During this 12-year period, the proportion of women who had been screened for GDM with a 50-g, 1-h oral challenge test during the second trimester increased by ~3% over time (age- and race/ethnicity-adjusted proportions 92.5 [95% CI 92.3–92.7] in 1995–1997 vs. 95.7 [95.6–95.9] in 2004–2006). If results were abnormal (1-h plasma glucose levels ≥7.8 mmol/l [140 mg/dl]), this test was followed by a standard diagnostic 100-g, 3-h OGTT. We identified 14,448 pregnancies that had a diagnosis of GDM from a health provider and with plasma glucose results during the index pregnancy that met the National Diabetes Data Group (NDDG) criteria on the 3-h 100-g OGTT for GDM, i.e., ≥2 glucose values at or exceeding the following thresholds: fasting, 105 mg/dl; 1 h, 190 mg/dl; 2 h, 165 mg/dl; and 3 h, 145 mg/dl (12,13) without recognized preexisting diabetes (14). We included only women who met the NDDG criteria of GDM because, in this clinical setting, the NDDG criteria were used for diagnosis of GDM until January 2007. We also required that these women had a diagnosis of GDM. Approximately 550 women had no diagnosis of GDM but met the NDDG criteria (equivalent to 4% of the size of our GDM cohort with a diagnosis); women without a diagnosis of GDM were excluded from the analysis.

For the first and second aims, the primary outcome was performance of postpartum screening for diabetes by either an FPG test alone or a 75-g, 2-h OGTT. Because several weeks may elapse before glucose metabolism returns to normal in most women with GDM (15), the American Diabetes Association (16) and the American College of Obstetricians and Gynecologists (17) both recommend that postpartum glucose screening be performed at 6 weeks postpartum or later. We considered the postpartum screening performed only if it was done during the first year after delivery starting from 6 weeks and if the woman was not pregnant again. For the third aim, the outcome was the proportion of women identified with either IFG (defined as an FPG ≥100 mg/dl but <126 mg/dl) or diabetes diagnosed by FPG ≥126 mg/dl (18). Among women who had postpartum screening, the proportion receiving the OGTT changed over time. Therefore, when we report on trends in postpartum conversion to diabetes, we report only on the fasting values detected by either an FPG or an OGTT. For the fourth aim, we examined the proportion of women who were identified as having pre-diabetes on the postpartum screen (that included IFG as defined above and impaired glucose tolerance [IGT] defined as a 2-h plasma glucose value ≥140 mg/dl) or diabetes (defined as an FPG >126 mg/dl or a 2-h plasma glucose value ≥200 mg/dl) (18).

Age, gestational age at delivery, race/ethnicity, maternal body weight during the beginning of the second trimester, and gestational age at GDM diagnosis were ascertained from the computerized medical records at birth. Use of insulin or glyburide during pregnancy was obtained from the pharmacy database. Because we had data on body weight but not on height, a woman was considered obese if her weight was ≥90th percentile of the weight distribution of women of her race/ethnicity in this study population. Infant birth weight was obtained from the electronic medical record. Macrosomia was defined as birth weight >4,000 g. Data on annual household income were based on census block data. Education and parity were obtained by linkage with the state of California birth certificate database. Because the lag time before state birth certificates became available is ~3 years, we have these variables for women who delivered between 1995 and 2004.

As in other reports (5–8), the majority of women in our cohort did not undergo postpartum diabetes screening in the early years of the study. Asian and Hispanic women were more likely to undergo postpartum screening. It is possible that health care providers might have recommended more postpartum screening among these racial/ethnic groups, given their higher prevalence of diabetes (19). It is also possible that Asian women were more likely to have had a recent physical examination, giving the health care provider the opportunity to recommend screening, as suggested by racial/ethnic differences in access to care among GDM women (20). Similar to other reports, we found that greater contact with medical care, either through a postpartum visit or other contacts, was associated with greater postpartum screening and may have provided additional opportunities to perform screening (6,7). Women who were more likely to be screened also were older and had higher educational attainment. Although reasons are speculative, these women may have had greater awareness of their diabetes risk and the recommendation for screening. In women who were screened postpartum, GDM was diagnosed earlier in their index pregnancy, and they were more likely to have been treated with medications, which may have increased their and their provider's awareness of their diabetes risk. Glucose levels on the diagnostic 3-h OGTT during pregnancy were similar in women who were and were not screened, suggesting that these were not used to guide testing. As shown by others (21), there was a suggestion that some of the women with a history of GDM who might have had a higher risk of developing diabetes during the postpartum period, such as those who were obese or with higher parity, were less likely to perform postpartum screening.

The American Diabetes Association (16), the American College of Obstetricians and Gynecologists (17), and the Fifth International Congress Workshop for Gestational Diabetes (15) endorse the postpartum OGTT and FPG to different extents. If only FPG were used in postpartum screening, 74 (40%) cases of IGT and 16 (75%) cases of diabetes would have been missed. Kitzmiller et al. (19) reported that among 527 women with GDM, at postpartum 16.5% had isolated IGT, only 16% of women in whom diabetes was diagnosed met the criteria for both elevated FPG and 2-h values, and 21 of 25 women met the criteria for diabetes according to their 2-h values alone. Hunt and Conway (21) also reported that one-third of their postpartum GDM cohort undergoing the OGTT and who had diabetes or pre-diabetes had isolated 2-h elevations. Their results are very similar to those found in this study: 78 of 204 women compared with 41 of 117 (or 38% vs. 35%). Therefore, the greater convenience of the FPG needs to be weighed carefully against its decreased sensitivity, particularly among women with a history of GDM.

This report has several limitations. We were not able to distinguish whether the lack of screening occurred because of a lack of provider order or other reasons. Such a distinction might have implications for interventions to improve screening performance. However, provider orders for screening might occur only after negotiation with the patient, and a lack of provider order may, at least in part, reflect women's objections to the test. We defined obesity by using race/ethnicity-specific percentiles, rather than height-to-weight ratios, thus introducing the possibility for misclassification and artificially decreasing the association between obesity and screening to the null. Information on other confounders, such as family history of diabetes, was not available from electronic records.

Because the population of women with GDM is of reproductive age, postpartum screening and subsequent diagnoses of diabetes affect not only the mothers but also future pregnancies. The risk of complications, particularly stillbirths and congenital abnormalities, may be reduced with optimal glycemic control before the subsequent pregnancy (1). Prepregnancy glycemic control might also reduce the risk of the infant to the in utero exposure to hyperglycemia that might lead to childhood obesity and diabetes (22). A diagnosis of pre-diabetes would identify women at high risk of future maternal diabetes, but this risk could be reduced through the application of interventions such as thiazolidinediones, metformin, or intensive lifestyle modification (3,4).