PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of canjcardiolThe Canadian Journal of Cardiology HomepageSubscription pageSubmissions Pagewww.pulsus.comThe Canadian Journal of Cardiology
 
Can J Cardiol. 2009 August; 25(8): e284–e287.
PMCID: PMC2732383

Language: English | French

Incidence of gestational hypertension in the Calgary Health Region from 1995 to 2004

Robin L Walker, MSc,1 Brenda Hemmelgarn, PhD MD,1,2 and Hude Quan, PhD MD1,3

Abstract

BACKGROUND

Hypertension is one of the most common medical conditions complicating pregnancy, and is a major contributor to maternal, fetal and neonatal morbidity and mortality in Canada.

OBJECTIVES

To determine the incidence and trends of gestational hypertension among pregnant women in the Calgary Health Region of Alberta.

METHODS

Gestational hypertension was classified according to current published Canadian guidelines: without proteinuria and with proteinuria. Hospital discharge abstracts were used to identify women diagnosed with gestational hypertension in the Calgary Health Region between January 1995 and December 2004. The Birth Registry of Vital Statistics was used to determine the number of live births and stillbirths in the Calgary Health Region during the study period. Incidence was calculated with 95% CIs. Age-standardized gestational hypertension rates were calculated using 2004 live births and stillbirths as the reference.

RESULTS

Over the 10-year period, the incidence of nonproteinuric gestational hypertension was relatively stable even after age adjustment, with an average incidence of 6.3% (95% CI 6.1% to 6.4%). When gestational hypertension was stratified by age, women aged 35 years or older had an almost twofold higher incidence of both nonproteinuric and proteinuric gestational hypertension than those younger than 35 years of age.

CONCLUSIONS

Interventions to prevent and improve the management of gestational hypertension, particularly among women aged 35 years and older, are required.

Keywords: Alberta, Gestational hypertension, Incidence, Pregnancy

Résumé

HISTORIQUE

L’hypertension est l’une des principales complications médicales de la grossesse et contribue de manière importante à la morbidité et à la mortalité maternelles, fœtales et néonatales au Canada.

OBJECTIF

Analyser l’incidence et les tendances de l’hypertension gestationnelle chez les femmes enceintes de la région sanitaire de Calgary en Alberta.

MÉTHODE

Les auteurs ont classifié l’hypertension gestationnelle conformément aux directives canadiennes actuelles: sans et avec protéinurie. Les registres de sorties des hôpitaux ont permis de recenser les femmes ayant reçu un diagnostic d’hypertension gestationnelle dans la région sanitaire de Calgary entre janvier 1995 et décembre 2004. Le registre des naissances du Bureau de l’état civil a servi à déterminer le nombre de naissances vivantes et le taux de mortinatalité dans la région sanitaire de Calgary durant la période couverte par l’étude. L’incidence a été calculée avec des IC à 95 %. Les taux d’hypertension gestationnelle standardisés selon l’âge ont été calculés en fonction du nombre de naissances vivantes et du taux de mortinatalité de 2004 comme référence.

RÉSULTATS

Au cours de cette période de dix ans, l’incidence de l’hypertension gestationnelle non protéinurique a été relativement stable, même après ajustement pour tenir compte de l’âge, avec une incidence moyenne de 6,3 % (IC à 95 %, 6,1 % à 6,4 %). Lorsque l’hypertension gestationnelle a été stratifiée selon l’âge, on a constaté une incidence deux fois supérieure d’hypertension gestationnelle non protéinurique et protéinurique, chez les femmes de 35 ans et plus comparativement aux femmes de moins de 35 ans.

CONCLUSION

Des interventions s’imposent pour prévenir l’hypertension gestationnelle et améliorer sa prise en charge chez les femmes de 35 ans et plus.

In Canada, one-quarter of the adult population has hypertension, and it is estimated that 90% of those who live an average lifespan will develop the disorder (1). Hypertension is also one of the most common medical conditions complicating pregnancy, and is a major contributor of maternal, fetal and neonatal morbidity and mortality (2). An estimated one-third of all maternal deaths in Canada are caused by hypertensive disorders, a trend that has changed little since the early 1970s (2,3). Pregnant women with hypertension, either newly diagnosed or pre-existing, are at risk for severe complications such as placental abruption, cerebrovascular accident, end-organ failure and disseminated intravascular coagulation (4,5). Also, the fetus is at risk for intrauterine growth restriction and prematurity (2).

There are several clinical reports and descriptive studies on hypertension and pregnancy-related disorders (2,3,69). While data from the United States have indicated a 5.9% incidence of gestational hypertension (10), to the best of our knowledge, no published studies have identified the magnitude and trends of gestational hypertension in Canada. The purpose of the present study was to determine the incidence and trends of gestational hypertension among pregnant women in a large Canadian health region.

METHODS

Gestational hypertension was classified as nonproteinuric or proteinuric (pre-eclampsia and eclampsia) according to current published Canadian guidelines (2).

Data sources

Canada’s universal health insurance program provides health care to residents of Alberta (one of 10 provinces in Canada) and their dependents. This program covers more than 99% of Alberta residents. As a result of this health insurance program, details regarding health care delivery – including patient demographics, hospitalizations and records of all births in the province – are collected and maintained by the Alberta government (Alberta Health and Wellness). These comprehensive population-based administrative databases were used in the present study to define gestational hypertension cases and identify the population at risk.

Numerator

The hospital discharge abstract database was used to identify women diagnosed with gestational hypertension in the Calgary Health Region (catchment population 1.1 million) between January 1995 and December 2004. Data on discharge diagnoses were coded in International Classification of Diseases (ICD), Ninth Revision, Clinical Modification (ICD-9-CM) until fiscal year 2002/2003 when a transition was made to the Canadian Enhancement to ICD, 10th Revision (ICD-10-CA). Cases were identified by the ICD-9-CM (nonproteinuric 642.0, 642.3, 642.4; proteinuric 642.5, 642.6) or ICD-10-CA (nonproteinuric O13; proteinuric O14, O15) discharge diagnosis in any of the 16 fields indicating gestational hypertension. Women younger than 14 years of age or older than 50 years of age, those living outside the Calgary Health Region, and those diagnosed with pre-existing hypertension or hypertension secondary to another disease (ICD-9-CM [642.1, 642.2, 642.7, 642.9] and ICD-10-CA [O10, O11]) were excluded. The hospital discharge abstract database captures all discharges from hospitals in the region regardless of patients’ location of residence. An individual’s residence was determined based on their six-digit postal code.

Denominator

In Alberta, it is mandatory to report all live births and stillbirths to the Birth Registry of Vital Statistics. The registry contains detailed information on both mothers and babies. The number of live births and stillbirths in the Calgary Health Region during the study period were determined from this registry.

Although the validity of defining gestational hypertension based on discharge diagnosis codes has not been assessed, it is likely that its validity is high. Canadian hospital discharge data have been validated in the past. Quan et al (11,12) conducted validation studies on comorbidities and clinical conditions, and found that administrative data are accurately coded for many severe or life-threatening conditions such as myocardial infarction, diabetes and cancer, but that some clinically nonspecific and symptomatic conditions such as weight loss and rheumatological disease, are less accurately coded. Gestational hypertension is a severe condition for obstetric patients, and thus would meet the criteria as being in the high-validity category (13).

Analysis

Incidence was calculated by dividing the number of women diagnosed with gestational hypertension (nonproteinuric and proteinuric) by the number of live births and stillbirths in the Calgary Health Region (numerator and denominator described above). Descriptive statistics were used to calculate gestational hypertension incidence with 95% CIs using Fay and Feuer’s method (14). The incidence was then stratified by age using 35 years as a cut-off, because pregnancy at older ages has been associated with an increased risk of various maternal complications (15); thus, they are clinically considered to be a high-risk group. Pearson’s χ2 test was used to make statistical comparisons between the stratified age groups. Finally, the incidence was standardized using the age composition of women with live births and stillbirths in 2004 as the reference.

RESULTS

There were 133,118 live births and stillbirths, with 8321 and 850 women diagnosed with nonproteinuric and proteinuric gestational hypertension, respectively, in the Calgary Health Region between 1995 and 2004. The average age of these women was 30 years and the average length of stay in the hospital was 3.4 days. Over the 10-year period, the incidence of nonproteinuric gestational hypertension was relatively stable, even after age adjustment, with an average incidence of 6.3% (95% CI 6.1% to 6.4%) (Table 1). The average incidence of proteinuric gestational hypertension was 0.6% (95% CI 0.5% to 0.7%) with no significant changes after adjustment for age. From 1995 to 2001, the incidence of proteinuric gestational hypertension remained relatively stable, with an increase in 2002 from 0.4% (95% CI 0.3% to 0.5%) to 0.9% (95% CI 0.7% to 1.0%), an increased rate that persisted until 2004 (P<0.01). Multiple births in the Calgary Health Region remained stable throughout the study period (1.6% in 1994 and 1.7% in 2004) and did not affect incidence rates.

TABLE 1
Crude and age-standardized nonproteinuric and proteinuric gestational hypertension incidence among live births and stillbirths in the Calgary Health Region

When gestational hypertension was stratified by age, compared with those younger than 35 years of age, those 35 years of age or older had an increased incidence of both nonproteinuric (5.8% [95% CI 5.7% to 5.9%] versus 9.2% [95% CI 8.7% to 9.6%]) and proteinuric (0.6% [95% CI 0.5% to 0.6%] versus 1.1% [95% CI 1.0% to 1.3%]) gestational hypertension (Tables 2 and and3).3). When comparing the stratified age groups, there was a statistical difference between the total rate of nonproteinuric gestational hypertension (P<0.01) and the total rate of proteinuric gestational hypertension (P<0.01).

TABLE 2
Crude nonproteinuric and proteinuric gestational hypertension incidence among live births and stillbirths in the Calgary Health Region among those younger than 35 years of age
TABLE 3
Crude nonproteinuric and proteinuric gestational hypertension incidence among live births and stillbirths in the Calgary Health Region among those 35 years of age or older

DISCUSSION

Over the 10-year study period, the incidence of nonproteinuric gestational hypertension remained relatively constant (ranging from 5.6% to 7.0%), even after adjusting for mother’s age. A slight increase was seen in the incidence of proteinuric gestational hypertension in 2002, which remained stable until the end of the study period. The incidence rates in the present study were slightly higher than those reported in the United States (9).

Despite advances in medical therapy in other areas over the past decade, these results suggest that there has been little, if any, progress in reducing the incidence of gestational hypertension, a condition that has significant adverse consequences to both the mother and the child. A potential reason for the lack of progress is that effective interventions aimed at preventing this disorder have not been established. During the past decade, several randomized controlled trials (16,17) have assessed the effects of dietary, vitamin and pharmacological interventions – including protein or salt restriction, zinc, magnesium, fish oil, calcium, vitamin C and E supplementation, as well as low-dose acetylsalicylic acid – in women with various risk factors to reduce the rate or severity of pre-eclampsia. However, little emphasis has been placed on practical preventive measures for the earlier stages of pre-eclampsia, namely nonproteinuric gestational hypertension.

Although the incidence of gestational hypertension remained relatively constant when all women were considered, when stratified by age, we found that women 35 years of age or older had a nearly twofold higher incidence of both nonproteinuric and proteinuric gestational hypertension than women younger than 35 years of age. During the past 20 years, there has been a growing tendency for mothers to have their children later in life (15). In 2006, Statistics Canada (18) reported that births to older mothers (35 years of age or older) were almost four times as frequent as a generation earlier. These mothers accounted for 17.2% of births in 2004, nearly four times higher than the value of 4.6% one-quarter of a century earlier. Therefore, gestational hypertension among older mothers is a particularly important and relevant medical issue, with important public health implications, as the proportion of women 35 years of age and older giving birth in Canada continues to increase.

The incidence of proteinuric gestational hypertension remained relatively stable from 1995 to 2001, with an incidence of 0.6%, but started to increase in 2002, reaching 0.9% in 2004. The increase of proteinuric gestational hypertension may be the result of many unknown causes, but we could not exclude the influence of the coding system change from ICD-9-CM to ICD-10-CA in April 2001. However, there are no substantial changes between the two coding systems in grouping subtypes of gestational hypertension and coding descriptions for these codes. Although the validity of hospital discharge data in recording gestational hypertension has not been evaluated, a recent Canadian study (11) found similar validity of ICD-9-CM and ICD-10 administrative data in recording clinical conditions with few exceptions. Further study is needed to explore the underlying reason(s) for the increased incidence of proteinuric gestational hypertension in recent years.

Limitations

The present study has some limitations. First, results of our study are based on data from one large health region in Canada, which may affect the generalizability of our findings. Second, the study only included live births and stillbirths that occurred at hospitals and did not include births outside of hospitals, although this would account for a very small proportion of deliveries in Alberta. Third, we were unable to obtain data on abortions in private clinics, although this is unlikely to have affected the results because gestational hypertension develops after 20 weeks of gestation and only 0.4% of abortions in Canada occur after 20 weeks of gestation (19). Finally, misclassification of gestational hypertension (ie, noncoding of gestational hypertension) may have occurred, which would have biased our results toward underestimating the incidence of gestational hypertension, although it is reasonable to believe that this would be an uncommon event. The coding system change from ICD-9 to ICD-10 could have influenced the likelihood of misclassification bias changing over time, but because there were no substantial changes between the two coding systems in grouping subtypes and coding descriptions, we believe this would be minimal. We are unaware of any other changes (ie, hospital policy) that could have improved or worsened the accuracy of gestational hypertension coding.

CONCLUSION

We found that the incidence of gestational hypertension has remained relatively stable over the past 10 years. If the incidence rates in Calgary are nationally representative, then we project that there are more than 20,000 cases of nonproteinuric and more than 2000 cases of proteinuric gestational hypertension in Canada each year, which may have significant adverse effects on both the mother and child (20). Importantly, we found that the incidence of gestational hypertension varied considerably by maternal age, with the incidence among women aged 35 years or older almost twofold higher than among women younger than 35 years of age. This is particularly important as the proportion of women aged 35 years and older giving birth continues to increase. Interventions to prevent and improve the management of gestational hypertension, particularly among women 35 years of age and older, are required.

REFERENCES

1. Joffres MR, Hamet P, Rabkin SW, Gelskey D, Hogan K, Fodor G. Prevalence, control and awareness of high blood pressure among Canadian adults. CMAJ. 1992;146:1997–2005. [PMC free article] [PubMed]
2. Helewa ME, Burrows RF, Smith J, Williams K, Brain P, Rabkin SW. Report of the Canadian Hypertension Society Consensus Conference: 1. Definitions, evaluation and classification of hypertensive disorders in pregnancy. CMAJ. 1997;157:715–25. [PMC free article] [PubMed]
3. Moutquin J-M, Garner PR, Burrows RF, et al. Report of the Canadian Hypertension Society Consensus Conference: 2. Nonpharmacologic management and prevention of hypertensive disorders in pregnancy. CMAJ. 1997;157:907–19. [PMC free article] [PubMed]
4. Cifkova R. Hypertension in pregnancy. Vnitr Lek. 2006;52:263–70. [PubMed]
5. Fournier A, Fievet P, el Esper I, el Esper N, Vaillant P, Gondry J. Hypertension and pregnancy. Diagnosis, physiopathology and treatment. Schweiz Med Wochenschr. 1995;125:2273–98. [PubMed]
6. Ray JG, Vermeulen MJ, Schull MJ, Singh G, Shah R, Redelmeier DA. Results of the Recent Immigrant Pregnancy and Perinatal Long-term Evaluation Study (RIPPLES) CMAJ. 2007;176:1419–26. [PMC free article] [PubMed]
7. Wenman WM, Joffres MR, Tataryn IV. A prospective cohort study of pregnancy risk factors and birth outcomes in Aboriginal women. CMAJ. 2004;171:585–9. [PMC free article] [PubMed]
8. Lee W, O’Connell CM, Baskett TF. Maternal and perinatal outcomes of eclampsia: Nova Scotia, 1981–2000. J Obstet Gynaecol Can. 2004;26:119–23. [PubMed]
9. Sibai BM. Diagnosis and management of gestational hypertension and preeclampsia. Obstet Gynecol. 2003;102:181–92. [PubMed]
10. Zhang J, Meikle S, Trumble A. Severe maternal morbidity associated with hypertensive disorders in pregnancy in the United States. Hypertens Pregnancy. 2003;22:203–12. [PubMed]
11. Quan H, Li B, Saunders D, et al. Assessing validity of ICD-9-CM and ICD-10 administrative data in recording clinical conditions in a unique dually coded database. Health Serv Res. 2008;43:1424–41. [PMC free article] [PubMed]
12. Quan H, Parsons GA, Ghali WA. Validity of procedure codes in International Classification of Diseases, 9th revision, clinical modification administrative data. Med Care. 2004;42:801–9. [PubMed]
13. Canadian Institute of Health Information. Canadian Coding Standards for ICD-10-CA and CCI, 2007. Ottawa: Canadian Institute of Health Information; 2007.
14. Fay MP, Feuer EJ. Confidence intervals for directly standardized rates: A method based on the gamma distribution. Stat Med. 1998;16:791–801. [PubMed]
15. MacNab YC, Macdonald J, Tuk TA. The risks of childbearing at older ages. Health Rep. 1997;9:41–50.
16. Solomon CG, Seely EW. Hypertension in pregnancy. Endocrinol Metab Clin North Am. 2006;35:157–71. vii. [PubMed]
17. Sibai BM. Diagnosis, prevention, and management of eclampsia. Obstet Gynecol. 2005;105:402–10. [PubMed]
18. Statistics Canada. Births. < http://www.statcan.gc.ca/daily-quotidien/060731/dq060731b-eng.htm> (Version current at July 31, 2006)
19. Abortion Rights Coalition of Canada. Late Term Abortions. Vancouver: Abortion Rights Coalition of Canada; 2005. Position Paper #22.
20. Statistics Canada. Birth and birth rate, by province and territory. < http://www40.statcan.gc.ca/l01/cst01/demo04a-eng.htm> (Version current at May 7, 2007)

Articles from The Canadian Journal of Cardiology are provided here courtesy of Pulsus Group