Background
Adverse pregnancy outcomes such as gestational diabetes mellitus and preeclampsia have important consequences on the growth, development, and health of children and mothers alike. Gestational diabetes mellitus is defined as carbohydrate intolerance with onset or recognition during pregnancy [
1]. The true prevalence of gestational diabetes is unknown, with estimates ranging from 1 to 14 % of pregnancies affected in the United States annually [
2‐
4]. Preeclampsia is a pregnancy complication that is characterized by the onset of hypertension and the presence of protein in the urine at >20 weeks gestation in a previously normotensive woman [
5]. It is estimated to have affected 3.8 % of US deliveries in 2010, with the rate of severe preeclampsia rapidly increasing over the past three decades [
6].
Despite the fact that the incidence of both gestational diabetes and preeclampsia is on the rise, there is concern that these and other pregnancy complications are not systematically recognized, diagnosed, or reported. In fact, numerous studies have found significant inconsistency between medical information that is reported on birth certificates and other medical records such as hospital-discharge records. A recent study found that in Washington State, most pregnancy complications in general, and gestational diabetes in particular, are substantially underreported on birth certificates compared to hospital-discharge data [
7]. Another study conducted in Ohio discovered that birth certificate data was less reliable than hospital records for identifying maternal risk factors, comorbid conditions, and complications of pregnancy, labor, and delivery [
8]. Moreover, a number of other studies have discovered underreporting of birth defects [
9], delivery complications [
10], and other standard measures taken at birth [
8] using state birth certificates alone. A review of the current literature suggests that birth certificates are generally not reliable sources of information on tobacco and alcohol use, prenatal care, maternal risk, pregnancy complications, labor, and delivery [
11].
In addition to the question of systematic underreporting of adverse pregnancy outcomes, there is considerable interest in determining if this underreporting varies according to socioeconomic status. The socioeconomic gradient in preeclampsia is well established, with more recent data suggesting that there is a significant negative association between socioeconomic status and preeclampsia. Studies have found such an association using maternal education level [
12], census tract income level [
13], and household income level [
5] as socioeconomic indicators. However, inconsistent findings have also been reported, with older studies finding no association between low socioeconomic status and preeclampsia [
14‐
16].
Studies that have investigated the association between socioeconomic status and gestational diabetes, however, have generated conflicting results that vary according to the socioeconomic indicators used. Numerous studies have found low socioeconomic status to be associated with a higher risk of gestational diabetes using maternal employment status [
17], neighborhood income level [
18,
19], education level [
20,
21], type of hospital services used [
22], and family income level [
23,
24] as socioeconomic indicators. In contrast, a study that measured a combination of indicators including maternal employment status, education level, parity, and monthly income level, found no association between low socioeconomic status and gestational diabetes [
25]. Other studies have also found no association using neighborhood deprivation level [
26], insurance status [
27], and level of maternal education [
28] as markers.
It has not yet been determined if the discrepancies in the results of these studies may be accounted for by differential reporting rates among populations of varying socioeconomic levels, a feature that may vary across study settings. Additionally, the reliability of California’s birth certificate registry compared to available medical records has yet to be determined. Thus, this paper examines the possibility of underreporting of adverse pregnancy outcomes in patients in Los Angeles County and Orange County, California, using data from both birth certificates and hospital birth records, with a specific focus on possible differential rates of underreporting according to socioeconomic status.
Discussion
We found that the birth certificate data that was analyzed significantly underreported the incidences of preeclampsia and diabetes compared to the Memorial data, which was collected specifically for research purposes. In addition, the degree of underreporting was disproportionately distributed across groups of different socioeconomic status, with certain socioeconomic indicators exhibiting higher degrees of underreporting.
The degree of underreporting of both preeclampsia and diabetes using birth certificate data was significantly higher women with lower education levels compared to women with higher levels of education, in Hispanic women compared to non-Hispanic White women, in women with public insurance compared to those with private insurance. These results indicate a disparate underreporting problem in low socioeconomic groups for pregnancy complications when race, level of education, and insurance status are used as socioeconomic indicators.
Several reasons may exist to explain the discrepancies between our two datasets, such as the weaknesses of birth certificate data discovered by several other studies. These include inadequate auditing of birth certificate data by individual hospitals, variations in data collection, diagnosis, and reporting procedures across hospitals, the use of nonclinical or untrained personnel to record data, and budgetary restrictions that prevent state agencies from thoroughly assessing and ensuring the quality of birth certificate data [
8,
10,
35‐
38]. The Memorial data, in turn, may have had better quality on pregnancy complications because it was a research database that underwent more stringent quality checks by nurses. Although the Memorial database is not a gold standard, it is believed that it is more accurate than birth certificate data because information is recorded when patients are physically present and able to verify records.
We attempted to maximally match the Memorial and birth certificate records. However, there were still 8.47 % and 5.74 % of the Memorial records that could not be matched to the birth certificates due to missing matching variables (i.e. mother’s name and mother’s date of birth) and likely moderate to serious problems in misspelling of the names, respectively. We found differences in the matched and unmatched groups in the rates of preeclampsia and diabetes and in socio-demographic parameters. But since the patterns of difference in the matched and the unmatched groups was consistent between the Memorial data and the birth certificate data, we do not expect it to change our main conclusion of the underreporting problem in the birth certificate data.
Since the Memorial data did not differentiate between gestational diabetes and diabetes when reporting pregnancy outcomes, it was not possible to investigate gestational diabetes specifically in this study. However, because the birth certificate data began reporting gestational diabetes separately starting in 2006, we were able to perform two separate analyses on diabetes using 2001–2005 and 2001–2006 as periods of interest. Both periods showed the same patterns of underreporting of diabetes, and we thus suspect that our findings on total diabetes likely hold for gestational diabetes as well. It has been estimated that approximately 90 % of pregnancies that are complicated by diabetes mellitus represent women with gestational diabetes mellitus [
39]. In the 2006 birth certificate data in California, we observed that gestational diabetes accounted for 72 % of the total diabetes (Table
2). This high degree of overlap between gestational diabetes and diabetes during pregnancy further suggests that the findings of this study regarding diabetes in general may also be applicable to cases of gestational diabetes in particular.
These results are consistent with previous studies, particularly those that determined that birth certificates are not reliable sources of information regarding preeclampsia, gestational diabetes, and other maternal complications and characteristics, particularly when compared to hospital discharge records [
7‐
11]. Thus, our conclusion that the birth certificate database used in this study underreported the incidence of preeclampsia and gestational diabetes is supported by similar patterns found elsewhere in the United States. However, to our knowledge this is the first study to assess the reliability of hospital data and birth certificates in southern California, and the first to address differential reporting of preeclampsia and diabetes during pregnancy by socioeconomic status in the United States.
The socioeconomic differences seen in the underreporting of preeclampsia and gestational diabetes as specific outcomes of interest is a unique observation that has not been studied in southern California. However, similar results have been found by studies that have analyzed related, though not identical, variables elsewhere in the United States. Most notably, a study on the sensitivity of birth certificate reports of birth defects in Atlanta found that Non-Hispanic Black maternal race/ethnicity and maternal education levels lower than high school were independently associated with a lower probability of a birth defect diagnosis being reported on a birth certificate [
9]. The authors of this study hypothesized that this observation might be explained by disparities in access to healthcare, as well as variations in personnel and birth certificate completion procedures across hospitals. Although our study did not analyze birth defects, the underreporting of adverse pregnancy outcomes we found according to racial and education level factors followed a similar pattern and can be explained by the same observations.
Further research must be performed to elucidate an explanation for the poor reliability of this particular set of birth certificate data for pregnancy complications, as well as the observed socioeconomic gradient in underreporting of such outcomes. Nevertheless, these findings have important implications for future public health research. Studies that rely solely on birth certificate data to draw conclusions regarding pregnancy complications should be aware of a potential bias towards underestimating the incidence of these conditions, particularly in low socioeconomic groups. This is critical for the descriptive study of socioeconomic disparities in pregnancy complications, and might contribute to explain why discrepant results were reported in the past [
17‐
28], beside any true difference in disparities across study settings. Such biases are also critical for etiologic research studying the relationships between pregnancy complications and potential risk factors, especially when these are unevenly distributed according to socioeconomic status. For instance, exposure to most air pollutants (e.g., primary particles from road traffic) is typically higher in populations with low socioeconomic status than in better-off ones [
40,
41]. In such a situation, a higher underreporting of maternal complications in populations with lower socioeconomic status would create a downward bias while measuring the association between air pollution and pregnancy complications. Consequently, researchers should attempt to use high quality health outcome data such as the Memorial database, either in place of or in conjunction with birth certificate data, whenever possible in order to minimize bias.
Furthermore, these findings indicate that there is a considerable need to improve the quality of birth certificate data in California, as far as pregnancy complications are concerned. There is a possibility that the quality of birth certificate has improved since 2006, the last year of this analysis. It would be beneficial to assess the quality of current birth certificate data in order to identify areas that still require improvement. However, historical birth certificate data are still of high importance for research studies that examine the impact of in-utero exposure on various long-term health effects (e.g. cognition and school performance in children, obesity, cardiovascular diseases etc.). Standardizing data collection and reporting procedures across hospitals would help minimize the discrepancies seen between birth certificate data and hospital databases such as the Memorial database. Because diabetes and preeclampsia are conditions that are oftentimes diagnosed prior to delivery and not at the hospital of delivery, there is also a need to improve the integration of prior medical records from other sources with hospital and birth certificate records. What is more, the fact that the birth certificate data underreported both preeclampsia and diabetes and did so to a higher degree among groups of lower socioeconomic status suggests that it would be most effective to focus standardization efforts on these particular conditions and among these identified groups, including Black and Hispanic women, women with lower levels of education, and women with public insurance. Finally, the most disadvantaged women may not have access to health care; thus improving health care access for low-income and minority people may also improve the reporting of pregnancy complications.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
JW conceived the study, designed the methods, conducted part of the analyses in the first draft and most of the analyses in the revision. NH helped with the analyses and drafted the manuscript. MH and OL contributed to methods and data analysis. JC retrieved hospital-based birth record data. PN helped with literature review. All authors contributed to the interpretation of data and edited various sections of the manuscript. All authors read and approved the final manuscript.