Maternal characteristics and pregnancy outcomes of women with chronic hypertension: a population-based study

Introduction

Maternal chronic hypertension (CHTN) is a risk factor in pregnancy, which is becoming more common as women become pregnant in their later years. Chronic or pre-existing HTN is defined as HTN that antedates pregnancy, is present before the 20^th week of pregnancy or persists longer than 12 weeks postpartum [1]. CHTN complicates 3%–5% of pregnancies [2]. The incidence is gradually rising due to various contributory factors such as increasing rates of obesity, pregnancy in advanced age and sedentary lifestyles.

Pregnancy complicated with CHTN is associated with adverse maternal and fetal outcomes with increased rates of pre-eclampsia (PET), abruptio placenta, hospitalization, adverse effect on fetal growth with iatrogenic preterm delivery and its consequences.

Despite the known complications of CHTN, establishing a diagnosis of CHTN in pregnancy is sometimes challenging as the physiological changes that occur during pregnancy especially in the second trimester reduce the blood pressure to be within the normal range which will erroneously label the patient as normotensive especially if the patient is booked during the second trimester of pregnancy. Therefore, a subset of patients are diagnosed as gestational hypertensive once the physiologic reduction in blood pressure ends in the third trimester of pregnancy [1], [3]. This may also lead to a lost opportunity for close monitoring in order to prevent the consequences associated with CHTN in pregnancy on both the fetal and the maternal health.

There are many studies on gestational HTN and PET, but there is a lack of significant international data or data from the Middle East on CHTN and its effect on pregnancy especially among a multiethnic population. This article aims to provide an understanding of maternal characteristics, perinatal outcomes and the current trend in CHTN in pregnancy when compared to non-hypertensive mothers from a middle-eastern perspective.

Materials and methods

Results

We identified 223 births of mothers with CHTN. The overall prevalence of CHTN in our population was 1.1% (223/20,210). The majority of cases were in singleton gestations (219/19,762) also giving us a prevalence rate of 1.1%.

The maternal characteristics are presented in Table 1. There was no significant difference in the nationality of those women with CHTN and those without, 71.7% (157/219) of women with CHTN were non-Qatari compared to 74.4% (14,474/19,452) in the non-CHTN group (P=0.36). There was a significant difference observed between women with CHTN and those without for age, BMI, parity, pre-existing and gestational diabetes and previous poor obstetric history. Maternal age at delivery ≥35 years was significantly more common in the CHTN group [129/219 (58.9%) vs. 3648/19,543 (18.7%); P<0.001]. The early pregnancy BMI was only recorded in 27% (5296/19,762) of the cohort; however, women in the CHTN group were significantly more likely to be obese with a BMI more than 30 [55/86 (64%) vs. 1695/5210 (32.5%); P<0.001] compared to the non-CHTN group. Women with CHTN were also significantly more likely to be parous when compared to non-CHTN women [180/219 (82.2%) vs. 13,991/19,539 (71.6%); P<0.001], and more likely to have pre-existing diabetes [33/219 (15.1%) vs. 367/19,543 (1.9%); P<0.001] and gestational diabetes [79/219 (36.1%) vs. 5102/19,543 (26.1%); P<0.001] than the non-CHTN women.

The rate of stillbirth in a previous pregnancy was noted to be more common in CHTN group [7/219 (3.3%) vs. 280/19,543 (1.4%)] when compared to non-CHTN women (P=0.028).

Tables 2 and 3 demonstrate the perinatal outcomes for the two comparative groups. There was a significantly higher cesarean section rate in women with CHTN [122/219 (55.7%)] compared to non-CHTN women [6025/19,543 (30.8%) (P<0.001)]. There was no difference in PPH between the two groups [5% (11/219) vs. 5.1% (988/19,543)].

The incidence of major maternal morbidity associated with acute organ failure, disseminated intravascular coagulation or uncontrolled blood pressure requiring ICU admission was low in both groups (0.1% in the CHTN group and 0.5% in the non-CHTN group). There was a maternal death in the chronic hypertensive group due to superimposed PET with severe features.

With regard to neonatal outcomes, preterm birth (24–36 weeks) occurred in 26% (57/219) of CHTN women compared to 8% (1569/19,543) of women with no CHTN (P<0.001). Similarly, more neonates of CHTN women were admitted to the NICU than those of non-CHTN women (26.9% vs. 13.4%; P<0.01). Macrosomic infants were less likely to occur with women with CHTN (5.1% vs. 1.8%; P=0.03). The rate of stillbirth was similar between the CHTN group [2/219 (0.9%)] and the non-CHTN group [114/19,543 (0.6%)].

Discussion

Hypertensive disorders in pregnancy are a wide spectrum of disorders and still a complex disorder in pregnancy despite recent advances in screening, monitoring and treatment. Definition of certain hypertensive disorders in pregnancy such as PET and gestational HTN has gone through major changes over the last decade but there is no significant change in the definition of CHTN in pregnancy.

The prevalence of CHTN in pregnancy varies in different study populations but the worldwide prevalence rate is between 0.5% and 5% in pregnancies [2]. According to a nationwide sample analyzing the trends and prevalence of CHTN, 1995–1996 through 2007–2008 in the USA, it was noted that the prevalence is gradually increasing from 0.9% to 1.8%, respectively [5].

The incidence of CHTN in a population is influenced by the ethnicity and prevalence of modifiable risk factors such as obesity and smoking among the studied population. The prevalence of CHTN in our multiethnic population was 1.1% and it is comparatively less than the prevalence rates published by the studies from the Western population. This lesser prevalence rate may be because the majority of women in the gulf region conceive before 35 years of age and because the prevalence of smoking, drug abuse and intake of alcohol is very low in this population for a variety of social and cultural factors. The World Health Organization (WHO) Global Survey on Maternal and Perinatal Health performed in 2010 stated that the prevalence of CHTN in Qatar was 0.28% [6]. The results of our study, despite presenting a low incidence of CHTN amongst pregnant women, do suggest an upward trend in incidence of CHTN over the past 7 years since the WHO survey.

According to a secondary analysis of the WHO Global Survey on Maternal and Perinatal Health, one of the major risk factors for PET and eclampsia was CHTN [7]. It is therefore important that women with CHTN are known promptly, treated with appropriate medication and monitored closely for the wide spectrum of maternal and fetal signs and symptoms of superimposed PET as it is significantly associated with adverse perinatal outcomes.

CHTN can be classified into primary HTN, without an underlying cause, and secondary HTN, developed due to underlying causes, e.g. diabetes mellitus, autoimmune disorders and chronic kidney disease, with the prevalence of primary and secondary HTN reported at 88.8% and 11.2%, respectively [5]. We have reviewed the pregnancy outcomes of women with CHTN in general without sub-classifying HTN into primary or secondary based on causative factors.

Certain maternal characteristics such as age, BMI and ethnicity are known risk factors predisposing women to CHTN. Advancing maternal age has been found to be a strong predictor of CHTN. A large 5-year epidemiological study in the US concluded a strong association of CHTN with maternal age [8]. Similar results were seen in a prospective screening study in the UK where high booking blood pressure was associated with advancing maternal age [9].

The confirmation of CHTN in pregnancy results in an increased risk of adverse pregnancy outcomes. Superimposed PET, cesarean delivery, PPH and placental abruption all increased in pregnancies affected by CHTN, which can all impact maternal morbidity and mortality which may affect the quality of women’s long-term health.

In an analysis of outcomes for 763 women with CHTN enrolled into a multicenter trial of low-dose aspirin for the prevention of PET [10], the incidence of superimposed PET was 25%, a figure increased in women with more than 4-year history of CHTN. Panaitescu et al. [11] also reported a similar incidence of superimposed PET in women with CHTN, 22% [11].

The incidence of superimposed PET in our study is 7.5%, which is significantly less than the figure quoted internationally [10], [11]. We are unable to explain this low incidence. It may be related to the complex nature of our population, in terms of parity or ethnicity, and further prospective work is needed to explore this further, and identify any modifiable factors that can have adverse synergistic effects on patients with CHTN [11].

In this study, there was a near doubling of cesarean section rates amongst women with CHTN (56%) compared to non-hypertensive women (30%). Sibai et al. [12] in a comparative study of singleton term (>36 weeks) deliveries noted that pregnancies complicated with CHTN had statistically higher rates of cesarean deliveries [odds ratio (OR)=2.7; 95% confidence interval (CI) 2.4–3.0] [11]. Again similar associations between CHTN and cesarean delivery rates have been reported elsewhere [5] and are a result of high incidence of induced preterm delivery due to uncontrollable HTN, superimposed PET, IUGR and abruptio placentae.

Bateman et al. [5] reported that the incidence of pregestational diabetes among patients with CHTN was 6.6% compared to 0.7% in non-hypertensive women (P<0.001) and that this risk increased with lengthening duration of HTN over 12 years [5]. Whilst our incidence of diabetes is much higher, due in part to screening criteria, we have also seen an increase in diabetes in women with pre-existing HTN.

In a retrospective cohort study of 532,088 singleton pregnancies [8], the rate of preterm birth was 25.5%, while the rate of small for GA was 18.3% in patients with CHTN. The same study also reported that the rate of stillbirths was 2.5 times higher in the group with CHTN compared to controls. A very similar figure for an increased risk of stillbirth was also reported for women with CHTN. According to a prospective study, Yerlikaya et al. [9] examined 113,415 singleton pregnancies and found the OR of still birth to be increased (OR 2.6) in patients with CHTN [9]. Our study has demonstrated similar associations between preterm birth, still births and IUGR in addition to increased neonatal care admission in women with CHTN than those without.

Conclusion

It is advisable that women in the reproductive age group with CHTN be offered pre-pregnancy counseling, close monitoring of blood pressure, lifestyle modification such as diet and weight reduction and commencement of safe antihypertensive medications which can continue during any planned pregnancy. It would also be the time to investigate and determine whether the HTN is primary or secondary as management becomes much more complex requiring a multi-disciplinary team for those with secondary causes. Once pregnancy is confirmed, the importance of aspirin therapy, detection and monitoring of any new onset of albuminuria and the regular assessment of the fetal growth and well-being should be emphasized.

We have done a preliminary evaluation of incidence and risk factors associated with CHTN in pregnancy in a middle-eastern population. One notable finding is that superimposed PET was less common than described in previous studies in other parts of the world. This study will be the foundation on which we will be evaluate further the causes and co-morbidities of CHTN among this population. This study confirmed the depth of associated maternal and fetal adverse outcomes to enable targeted antenatal care for pregnant women with CHTN.