The most common national figures reported for GDM lie between 2 and 5%, whereas pregestational diabetes is said to affect ~1% of all pregnancies (2
). Other population-based investigations have also found increasing trends in GDM, consistent with our own findings (5
). With respect to trends for type 1 and type 2 diabetes among pregnant women, no other studies, to our knowledge, have documented national trends in pregestational diabetes, which we observed to increase throughout the length of the study period.
As expected, we found that older maternal age was an independent predictor of any diabetes among delivery hospitalizations. This association likely contributed to the observed increase in the overall rate of diabetes as the proportion of U.S. births to older women also increased (14
). Nonetheless, we found that diabetes rates among delivery hospitalizations increased for all ages, most sharply among the younger age-groups, noted for all three diabetes types. Other studies have documented similar findings, highlighting concerns about the rise of diabetes among younger women, a group previously thought to be at lower risk (7
). Additional factors that may be contributing to increases include improvements in screening and detection, as well as the rising prevalence of diabetes risk factors, such as obesity, poor diet, and inactivity, which are likely contributing to trends specifically in GDM and type 2 diabetes (15
With respect to type 1 diabetes, no clear trends have been previously established among adults. Pregnant women with pregestational diabetes have a much greater risk of maternal and fetal complications, including preeclampsia and congenital abnormalities than nondiabetic women (2
). Not surprisingly, in our study, hospitalizations coded with type 1 diabetes had the highest rates of cesarean delivery, the longest lengths of stay, and the highest total charges even after stratification by mode of delivery. Rising rates of type 1 diabetes, especially among the youngest groups, are a concern because of the potential for increased future burden of severe obstetric complications as these women age and become pregnant again.
As with type 1 diabetes, outcomes of type 2 diabetic pregnancies are also marked by an increased risk for fetal malformation and intrauterine death, as well as other obstetric complications (2
). Although pregnancies with pregestational diabetes have more adverse outcomes than those with GDM, perhaps the greatest significance of GDM is the increased risk of future development of type 2 diabetes that it confers (15
). As GDM prevalence increases, the number of women who enter subsequent pregnancies with pregestational diabetes, as well as the number of women who carry an increased lifetime risk of developing type 2 diabetes, is likely to increase.
Another factor that could have influenced diabetes trends in our study is changes to screening recommendations and diagnostic criteria that occurred during the study time frame. In 1997, an expert committee of the American Diabetes Association issued a report modifying the diagnostic criteria for diabetes, which resulted in the use of a fasting plasma glucose test of ≥126 mg/dl, rather than 140 mg/dl as the preferred tool and cutoff to diagnose type 2 diabetes (16
). The anticipated result of this modification was that an additional 2 million cases of diabetes would be diagnosed, many of which would be identified in younger individuals and as “early” type 2 diabetes (17
). Another modification that occurred in 2000 was the American Diabetes Association's adoption of the Carpenter and Coustan criteria for the diagnosis of GDM via the 75-g or 100-g oral glucose tolerance test (18
). Although use of the more conservative 1979 National Diabetes Data Group criteria was still supported by the American College of Obstetricians and Gynecologists, adoption of the Carpenter and Coustan criteria may have partially contributed to the increased detection of additional cases of GDM, particularly after 2000.
In addition to increased maternal age, we also demonstrated urban location to be significantly associated with all diabetes types. There is evidence of urban/rural disparities in diabetes prevalence, but contrary to our results, rural areas appear to carry a greater burden of the disease (19
). Reduced access to care in rural areas with the potential for underdiagnosis, together with our inability to adjust for potential drivers of urban/rural differences, including race and socioeconomic factors, may in part explain the associations we found with urban location.
We also demonstrated payer status, specifically Medicaid/Medicare, to be a significant predictor of a diagnosis of diabetes. Previous studies of other health outcomes using administrative data have used payer status as a crude proxy for socioeconomic status (SES) in the absence of other more commonly used markers (20
). Since enrollment in Medicaid is contingent on meeting low-income thresholds and because low SES is a well-documented risk factor for diabetes, this may partly explain the association we found between diabetes and Medicaid/Medicare payer status. Limitations, nevertheless, are present when using payer status as an SES marker, and thus results are interpreted with caution.
Finally, we also documented an association between southern region and a diabetes diagnosis at delivery for all types. According to national diabetes estimates, the U.S. South has the highest prevalence estimates in the country (15
). Similarly, obesity, a major risk factor for both GDM and type 2 diabetes, is also more pronounced in the South (22
). Regional disparities may also be a reflection of differences in screening, as well as differences in race, SES, and lifestyle factors, which, analogous to differences in urban/rural estimates, may account for the association found with region. Less clear, however, is an explanation for the regional differences that we also observed with type 1 diabetes. Some hypotheses have suggested that excess weight may accelerate development of type 1 diabetes, which could partly explain increasing trends (23
). Regional differences in obesity may also be reflected in the association with type 1 diabetes; however, because this hypothesis has not been thoroughly tested, explanations for the regional differences in type 1 diabetes remain unclear.
There are some limitations with this analysis. First, record identification with diabetes was based on discharge ICD-9-CM codes without knowledge of the criteria used to make the diagnosis. In general, studies that use ICD-9-CM codes to describe disease trends may suffer from bias, depending on the validity of the code for the condition being examined. A previous study that evaluated ICD-9-CM codes in hospital discharge data for use in obstetric research reported high positive predictive values (96%) and moderate sensitivity (64%) for the full spectrum of diabetes codes (24
). Similar results were reported in another study that assessed the validity of hospital discharge data for identifying diabetes-complicated births (25
). This result suggests the potential for underestimation rather than overreporting in our numbers but would not deter from our conclusions regarding the impact of diabetes among pregnant women in the U.S. Similarly, because of the nature of the data, we also cannot rule out improvement in reporting quality over time as a partial explanation for the temporal increases. Population-based studies of laboratory-based diagnoses of GDM over similar time intervals, however, also documented increasing trends similar to what we report (5
Second, because race is not consistently reported by all states in the NIS dataset, we elected to exclude any examination by race. To the extent that sampling and race reporting varied throughout the study period, interpretation of trends may be affected as it would with any unmeasured covariate (e.g., obesity). Previous studies of diabetes trends in pregnancy with more reliable race data, however, also demonstrated findings consistent with our own, even after adjustment for race (5
). As a result, although we cannot rule out the contribution of a possible shift in the racial distribution over time to overall increases in diabetes in pregnancy in the U.S., our results nevertheless remain consistent with those of previous studies that were able to account for race.
We report increasing trends in all diabetes types among all age-groups from a sample of delivery hospitalizations in the U.S. from 1994 to 2004. Overall trends were largely driven by those with a diagnosis for GDM, although the sharpest rate increases were found among those with type 2. Most concerning was the identification of a group of relatively young women who have an increased lifetime risk of future development of type 2 diabetes and obstetric complications with subsequent pregnancies. Given the potential for maternal and perinatal morbidity and mortality associated with diabetes, it will be important to monitor trends among the pregnant population to target prevention strategies to minimize both these risks and the anticipated burden on the health care system.