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Keywords:

  • Fetal death;
  • gestational age;
  • hypertensive disorders in pregnancy;
  • risk factors

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

Please cite this paper as: Ahmad A, Samuelsen S. Hypertensive disorders in pregnancy and fetal death at different gestational lengths: a population study of 2 121 371 pregnancies. BJOG 2012;119:1521–1528.

Objective  To compare the proportion of offspring that was stillborn in pregnancies with pre-eclampsia, gestational hypertension or chronic hypertension with those in normotensive pregnancies.

Design  Register-based observational study.

Setting  The Medical Birth Registry of Norway.

Population  All singleton births after 20 completed weeks of gestation in Norway from 1967 to 2006 (n = 2 121 371).

Methods  The proportion of stillborn offspring was estimated in normotensive pregnancies, and in pregnancies with pre-eclampsia, gestational and chronic hypertension at different gestational lengths. In addition, changes in the proportions of stillborn offspring by maternal hypertensive disorder from 1967–1986 to 1987–2006 were estimated.

Main outcome measures  Fetal death.

Results  The prevalence of hypertensive disorders in pregnancy was 4.7%. In total, 17 933 fetal deaths occurred and 9.2% of these were in hypertensive pregnancies. In normotensive pregnancies, 0.8% (16 290/2 022 400) experienced fetal death. This was true for 1.9% (1170/62 261) of the pregnancies with pre-eclampsia, 1.2% (390/32 068) with gestational hypertension and 1.8% (83/4642) with chronic hypertension. There was a 44% overall reduction in fetal death rate from 1967–1986 to 1987–2006. The largest decline was in women with pre-eclampsia (80% reduction). In women with gestational hypertension and chronic hypertension, the overall reductions in fetal death rates were 49% and 57%, respectively, comparable with the 41% decline in normotensive pregnancies.

Conclusions  In our nationwide study during 1967–2006, the risk of fetal death among women with hypertensive disorders in pregnancy has been greatly reduced, especially among pre-eclamptic women at term. The risk of fetal death among women with gestational or chronic hypertension has also decreased, but in a different manner.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

Hypertensive disorders are present in 5–10% of all pregnancies and, in these pregnancies, the risk of fetal death is increased.1–6 Hypertensive disorders in pregnancy include pre-eclampsia, gestational hypertension and chronic hypertension (hypertension prior to pregnancy). These conditions may have different risk factors and clinical manifestations, and therefore the influence on pregnancy outcomes may also differ.7–9

The contributions of hypertensive disorders to the risk of fetal death have not been studied thoroughly. It is known, however, that the risk of fetal death varies by the length of gestation and that the risk increases after 37 weeks of pregnancy.10–12 As the prevalence of maternal hypertensive disorders also increases with increasing gestational age,13 the increased risk of fetal death around term may be attributed in part to hypertension.

The overall risk of fetal death has decreased substantially during recent decades, particularly in term pregnancies.10 Moreover, in pregnancies with pre-eclampsia, the fetal death rate after 23 weeks of gestation has also decreased.14

In women with gestational hypertension or chronic hypertension, the risk of fetal death has been studied in lesser detail. However, the different hypertensive disorders of pregnancy may differentially alter the risk and timing of fetal death.9,15,16 An improved knowledge of the risk of fetal death by gestational age and hypertensive disorder is necessary for the prevention of such deaths.

Among all births in Norway during 1967–2006, we compared the overall proportions of offspring that were stillborn, and the proportions of stillborn offspring by gestational week in pregnancies with pre-eclampsia, gestational hypertension or chronic hypertension with those in normotensive pregnancies. We also studied changes in the prevalence of fetal death and perinatal mortality by maternal hypertensive disorder from 1967–1986 to 1987–2006.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

Study population

Data were obtained from the Medical Birth Registry of Norway.17 This registry was established in 1967 and includes information on offspring and maternal characteristics of all births in Norway after 16 weeks of gestation. The information is obtained from standardised forms. From 1967 to 1998, these forms had open text fields regarding maternal health that were completed by the attending midwife or doctor shortly after delivery. In 1999, the Medical Birth Registry of Norway introduced new forms with pre-coded fields for maternal disorders. Notification to the Medical Birth Registry is mandatory by law.

Between 1967 and 2006, there were 2 337 775 births in Norway; we included singleton births only in our study sample. Hence, multiple pregnancies (n = 60 498; 2.6%), pregnancies with missing length of gestation (n = 125 997; 5.4%), pregnancies recorded to last longer than 43 weeks (n = 28 595; 1.2%) or <20 weeks (n = 4325; 0.2%) and pregnancies with missing information on maternal age or parity (n = 53) were excluded. A total of 2 121 371 births was therefore included in our analysis.

Study factors

The outcome measure, fetal death, was defined as the birth of a dead offspring after 20 weeks of gestation. We studied the overall proportion of stillborn offspring and the proportions by gestational age at birth. Gestational age at birth was calculated from the date of the last menstrual period for births before 1999 and, thereafter, on estimations of term date at routine fetal ultrasonographic examinations at pregnancy week 17–19. Perinatal mortality was defined as the number of offspring deaths in pregnancies lasting ≥22 weeks (154 days) and within the first 7 days after birth per 1000 births.

Pre-eclampsia, gestational hypertension, chronic hypertension, eclampsia and HELLP (haemolysis, elevated liver enzymes and low platelets) are reported as separate categories to the Medical Birth Registry.18 Before 1999, the hypertensive diagnoses in the Medical Birth Registry were defined according to the International Classification of Diseases, Eighth Revision (ICD-8) and, thereafter, according to ICD-10. Maternal hypertension in pregnancy was grouped into three mutually exclusive categories of hypertensive disorders in our analysis: pre-eclampsia, gestational hypertension and chronic hypertension.5

Pre-eclampsia was defined as an increase in blood pressure to at least 140/90 mmHg combined with proteinuria after completion of 20 weeks of gestation (ICD-8 codes 637.4/637.5/637.6/637.9 and ICD-10 codes O13 and O14). Eclampsia was defined as pre-eclampsia with seizures. Eclampsia and HELLP were grouped together with pre-eclampsia. In addition, women with chronic hypertension who developed pre-eclampsia during pregnancy were grouped as pre-eclampsia.

Gestational hypertension was defined as an increase in blood pressure to ≥140/90 mmHg after the completion of 20 weeks of gestation, or an increase in systolic blood pressure of 30 mmHg or of diastolic blood pressure of 15 mmHg or more without concomitant proteinuria (ICD-8 codes 637.0/637.2 and ICD-10 code O16).

Chronic hypertension was defined as a pre-pregnancy systolic blood pressure of ≥140 mmHg or diastolic blood pressure of ≥90 mmHg, or an increase in blood pressure to these values before 20 weeks of gestation (ICD-8 codes 400–404 and ICD-10 codes I10/I11/I12/I13/I15/O10/O11). Women who were not in any of these categories were labelled as normotensive in our study.

We made adjustments for maternal age at delivery and parity, as these are known independent risk factors for fetal death, and as the distribution of parity and maternal age has changed over the time periods.19,20 Maternal age was categorised as ≤19, 20–24, 25–29, 30–34, 35–39 and ≥40 years at delivery. Parity was defined as the number of previous deliveries after 16 weeks of gestation, and categorised as 0, 1, 2, 3 and ≥4 deliveries.

Statistical methods

We calculated the overall proportion of deliveries with fetal death and the proportions with fetal death in each pregnancy week in pregnancies with pre-eclampsia, gestational hypertension, chronic hypertension and in normotensive pregnancies. We further studied the changes in fetal death rate among women with the different hypertensive disorders relative to the fetal death rate among normotensive women.The associations of the different hypertensive disorders with fetal death were estimated as crude and adjusted relative risks (RRs). For these analyses, we used the GENLIN (generalised linear model) command in SPSS 16.0 with binomial response and log-link. The uncertainty of the estimates is reported by 95% confidence intervals (95% CIs). Normotensive pregnancies were used as the reference. Adjustments were made for maternal age at delivery and parity. These analyses were also performed separately for deliveries in the two time periods: 1967–1986 and 1987–2006.

We used SPSS version 16.0 software (SPSS Inc., Chicago, IL, USA) for statistical analyses.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

Among all singleton pregnancies (2 121 371), there were a total of 17 933 fetal deaths (0.8%), and 9.2% (n = 1643) of these were in pregnancies with hypertensive disorders. Among the 2 022 400 women who were normotensive at delivery (95.3% of all pregnancies), 0.8% (n = 16 290) experienced fetal death (Table 1). In total, 62 261 women had pre-eclampsia (2.9% of all pregnancies), and 1.9% of these women experienced fetal death, giving an RR of 2.3 (95% CI, 2.2–2.5) with normotensive pregnancies as the reference. Among the 32 068 women with gestational hypertension (1.5% of all pregnancies), 1.2% experienced fetal death (RR, 1.5; 95% CI, 1.3–1.6). Likewise, among the 4642 women with chronic hypertension (0.2% of all pregnancies), there were 1.8% fetal deaths (RR, 2.1; 95% CI, 1.7–2.6) (Table 1).

Table 1.   Relative risks (RRs) with 95% confidence intervals (95% CIs) of fetal deaths in pregnancies with pre-eclampsia, gestational or chronic hypertension relative to normotensive pregnancies in Norway during 1967–2006
 Fetal deaths/birthsFetal deaths/1000 birthsCrude RR (95% CI)Adjusted RR* (95% CI)
  1. *Adjustments were made for maternal age and parity.

All17933/21213718.5  
Normotensive16290/20224008.1ReferenceReference
Pre-eclampsia1170/6226118.82.33 (2.20–2.47)2.29 (2.16–2.43)
Gestational hypertension390/3206812.21.51 (1.37–1.67)1.46 (1.32–1.61)
Chronic hypertension83/464217.92.22 (1.79–2.75)2.12 (1.71–2.63)

The proportion of offspring that was stillborn varied according to the length of gestation (Figure 1). In gestational week 28, 20.3% of all offspring were stillborn, whereas this was true for 0.2% of the offspring in gestational week 40 and 0.5% in gestational week 43. At term, the proportion of births with stillborn offspring was higher in hypertensive relative to normotensive pregnancies. In gestational week 41, 0.5% of births in hypertensive pregnancies were stillborn, whereas this was true for 0.2% in normotensive pregnancies (RR, 2.5; 95% CI, 2.0–3.2). After adjustment for maternal age and parity, the RR remained essentially the same: adjusted RR of 2.3 (95% CI, 1.8–3.0).

image

Figure 1.  Number of fetal deaths per 1000 births at each gestational week in pregnancies with pre-eclampsia, gestational or chronic hypertension and in normotensive pregnancies in Norway during 1967–2006.

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There was an overall decline in the fetal death rate in all hypertension groups during our study period; however the decline occurred earlier and was larger among pre-eclamptic pregnancies (Figure 2). In normotensive pregnancies, the proportion of stillborn offspring declined from 10.1 to 6.0 per 1000 births from 1967–1986 to 1987–2006 (41% reduction), whereas, in pregnancies with pre-eclampsia, the decline was from 36.2 to 7.2 per 1000 births (80% reduction) (Table 2). In 1967–1986, the adjusted RR of fetal death in pregnancies with pre-eclampsia was 3.4 (95% CI, 3.2–3.6) relative to normotensive pregnancies, whereas, in 1987–2006, the RR was reduced to 1.2 (95% CI, 1.0–1.3). When studying gestational weeks 40–43 separately, the adjusted RR of fetal death associated with pre-eclampsia was 3.0 (95% CI, 2.5–3.7) in 1967–1986, but 1.2 (95% CI, 0.8–1.8) in 1987–2006, using normotensive pregnancies as the reference (Table 3).

image

Figure 2.  Number of fetal deaths per 1000 births in pregnancies with pre-eclampsia, gestational or chronic hypertension and in normotensive pregnancies in Norway during 1967–2006.

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Table 2.   Relative risks (RRs) with 95% confidence intervals (95% CIs) of fetal deaths in pregnancies with pre-eclampsia, gestational or chronic hypertension relative to normotensive pregnancies in Norway during 1967–1986 and 1987–2006
 Fetal deaths/birthsFetal deaths/1000 birthsCrude RR (95% CI)Adjusted RR* (95% CI)
  1. *Adjustments were made for maternal age and parity.

1967–1986 (All)11414/105165410.9  
Normotensive10205/100850310.1ReferenceReference
Pre-eclampsia903/2493036.23.58 (3.35–3.83)3.39 (3.17–3.63)
Gestational hypertension271/1711915.81.57 (1.39–1.76)1.46 (1.29–1.64)
Chronic hypertension35/110231.83.14 (2.26–4.35)2.80 (2.02–3.88)
1987–2006 (All)6519/10697176.1  
Normotensive6085/10138976.0ReferenceReference
Pre-eclampsia267/373317.21.19 (1.05–1.35)1.17 (1.04–1.33)
Gestational hypertension119/149498.01.33 (1.11–1.59)1.29 (1.08–1.55)
Chronic hypertension48/354013.62.26 (1.70–3.00)2.12 (1.61–2.83)
Table 3.   Relative risks (RRs) with 95% confidence intervals (95% CIs) of fetal deaths in pregnancies with pre-eclampsia, gestational and chronic hypertension relative to normotensive pregnancies by weeks of gestation during 1967–1986 (A) and 1987–2006 (B)
(A) 1967–1986
Gestational weeksTotal fetal deaths (n = 11 414)Total births (n = 1 051 654)Pre-eclampsia (n = 903/24 930)Gestational hypertension (n = 271/17 119)Chronic hypertension (n = 35/1102)
Fetal deaths/birthsAdjusted RR* (95% CI)Fetal deaths/birthsAdjusted RR* (95% CI)Fetal deaths/birthsAdjusted RR* (95% CI)
  1. *Adjustments were made for maternal age and parity.

20–231716    219112/160.98 (0.74–1.30)6/90.85 (0.53–1.35)5/5 
24–271548   366656/981.35 (1.13–1.60)14/241.36 (0.96–1.91)7/121.46 (0.89–2.38)
28–311455   6820175/4801.71 (1.51–1.95)41/872.04 (1.62–2.56)5/171.36 (0.65–2.83)
32–352094 25 988299/18232.21 (1.97–2.47)59/4011.85 (1.45–2.35)9/621.76 (0.96–3.23)
36–392563346 373248/106433.26 (2.86–3.71)79/64071.69 (1.35–2.11)6/4581.73 (0.78–3.84)
40–432038666 616113/118703.02 (2.50–3.65)72/101912.20 (1.74–2.78)3/5481.61 (0.54–4.98)
(B) 1987–2006
Gestational weeks Total fetal deaths (n = 6519) Total births (n = 1 069 717) Pre-eclampsia (n = 267/37 331) Gestational hypertension (n = 119/14 949) Chronic hypertension (n = 48/3540)
Fetal deaths/births Adjusted RR* (95% CI) Fetal deaths/births Adjusted RR* (95% CI) Fetal deaths/births Adjusted RR* (95% CI)
20–231878254326/480.73 (0.56–0.95)9/140.87 (0.59–1.28)9/130.93 (0.64–1.33)
24–27946344561/4190.50 (0.39–0.64)16/421.32 (0.90–1.95)9/241.35 (0.80–2.26)
28–31701687556/15080.31 (0.24–0.41)15/1211.03 (0.64–1.66)8/501.27 (0.67–2.41)
32–3582527 48746/40520.35 (0.26–0.48)27/5581.47 (1.01–2.14)6/1751.02 (0.64–2.25)
36–391289406 00155/180350.95 (0.73–1.25)28/63251.37 (0.94–2.00)10/15541.99 (1.07–3.70)
40–43880623 36623/132691.19 (0.79–1.80)24/78892.05 (1.37–3.08)6/17242.30 (1.03–5.13)

In pregnancies with gestational hypertension, the overall decline in fetal death rate was from 15.8 per 1000 births in 1967–1986 to 8.0 per 1000 births in 1987–2006 (49% reduction). The adjusted RRs of fetal death in women with gestational hypertension were 1.5 (95% CI, 1.3–1.6) in 1967–86 and 1.3 (95% CI, 1.1–1.6) in 1987–2006 relative to normotensive pregnancies (Table 2). In deliveries at 40–43 weeks of pregnancy, the adjusted RRs of fetal death associated with gestational hypertension were 2.2 (95% CI, 1.7–2.8) in 1967–1986 and 2.1 (95% CI, 1.4–3.1) in 1987–2006.

In pregnancies with chronic hypertension, the fetal death rate declined from 31.8 to 13.6 per 1000 births (57% reduction) from 1967–1986 to 1987–2006. In these pregnancies, the adjusted RRs of fetal death were 2.8 (95% CI, 2.0–3.9) in 1967–1986 and 2.1 (95% CI, 1.6–2.8) in 1987–2006. However, as shown in Figure 2, there was a considerable decline in the fetal death rate among pregnancies with chronic hypertension after 1996. In deliveries at 40–43 weeks of pregnancy, the adjusted RRs of fetal death associated with chronic hypertension were 1.6 (95% CI, 0.5–5.0) in 1967–1986 and 2.3 (95% CI, 1.0–5.1) in 1987–2006.

The perinatal mortality also declined during the study period from 15.5 per 1000 births in 1967–1986 to 7.0 per 1000 births in 1987–2006 (Table 4). Among normotensive pregnancies, the rate declined by 54%, and the corresponding declines among pregnancies with pre-eclampsia, gestational hypertension and chronic hypertension were 79%, 51% and 62%, respectively.

Table 4.   Relative risks (RRs) with 95% confidence intervals (95% CIs) of perinatal mortality in pregnancies with pre-eclampsia, gestational and chronic hypertension relative to normotensive pregnancies during 1967–1986 and 1987–2006
 Perinatal mortalityPerinatal deaths/1000 birthsCrude RR (95% CI)Adjusted RR* (95% CI)
  1. *Adjustments were made for maternal age and parity.

1967–1986 (All)16330/105073415.5  
Normotensive14778/100759214.7ReferenceReference
Pre-eclampsia1162/2492746.63.18 (3.00–3.37)3.04 (2.86–3.22)
Gestational hypertension344/1711620.11.37 (1.23–1.52)1.30 (1.17–1.45)
Chronic hypertension46/109941.92.85 (2.15–3.79)2.68 (2.02–3.55)
1987–2006 (All)7451/10683967.0  
Normotensive6873/10126026.8ReferenceReference
Pre-eclampsia375/3731410.01.48 (1.34–1.64)1.46 (1.31–1.62)
Gestational hypertension147/149439.81.45 (1.23–1.71)1.42 (1.21–1.67)
Chronic hypertension56/353715.82.33 (1.80–3.03)2.23 (1.72–2.90)

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

In this study of more than 2 million singleton pregnancies in Norway during 1967–2006, women with hypertension showed an increased risk of fetal death. There was a 44% overall reduction in fetal death rate from 1967–1986 to 1987–2006. The largest decline was in women with pre-eclampsia (80% reduction). The fetal death rate among women with gestational and chronic hypertension also declined (49% and 57%, respectively). Hence, compared with the large decrease in fetal death among pre-eclamptic women, the RR of fetal death among women with gestational and chronic hypertension (relative to normotensive women) has declined in a different manner.

The strength of our study is that it included all singleton births in Norway after 20 weeks of gestation over 40 years. Almost all deliveries in Norway take place in public hospitals and the reporting of births is compulsory by law. Hence, we believe that there is no selection bias in our study. The diagnostic criteria for hypertensive disorders in pregnancy have remained unchanged during our study period. After the implementation of the new form with pre-coded tick boxes regarding maternal hypertensive disorders by the Medical Birth Registry of Norway, the incidence of all three categories of hypertension increased. This may be an indication of increased ascertainment, or there may have been a true increase as childbearing women in Norway have become older and may have become heavier. Increased reporting would most likely lead to increased registration of cases with milder disease, resulting in the attenuation of the risk estimates.

The prevalence of the different hypertensive disorders in our study is largely in agreement with other reports.21–26 Pre-eclampsia has been reported to complicate between 1.4% and 7% of all pregnancies,13,21,22,24,27 and gestational hypertension between 1% and 4.3% of all pregnancies.21,22,24,25 The prevalence of chronic hypertension varies between 0.2% and 5%, and the highest prevalence is among the oldest women.22–25,28 Discrepancies between rates are probably a result of different diagnostic criteria, different sources of information (birth records or hospital records) and differences in the source populations with regard to risk factors.29,30

The diagnosis of hypertensive disorders in our study was based on blood pressure measurements and urine examinations in antenatal care from 8 to 12 weeks of pregnancy and onwards.31 Close to 100% of pregnant women in Norway take part in the public antenatal care programme which is free of charge, and the standard number of examinations in the programme is ten.31 Maternal disease and clinical findings at each antenatal examination are recorded prospectively and reported to the Medical Birth Registry shortly after delivery. Differential misclassification of hypertensive disorders in pregnancy according to offspring vital status at birth is therefore unlikely. Furthermore, misclassifications are more likely to underestimate than overestimate associations.

We lacked information on body mass index, which is a known risk factor for both hypertension and fetal death.32 As women with high body mass index more often have hypertension, our estimated association of hypertension with fetal death may represent overestimates.33 However, a differential association of high body mass index with type of hypertension and fetal death is unlikely.

Previous studies on a possible differential association of type of hypertension with fetal death report diverging estimates, and most studies are small.26,34 Population studies on the association of hypertension with fetal death from the USA have been published, but discrimination between types of hypertension in pregnancy could not be made.1,16,35 One of these studies reported changes from 1990–1991 to 2003–04, and found an increase in pregnancy-induced hypertension (from 3 to 3.8%) and a decrease in stillbirth rate (from 6.1 to 4.8 per 1000 births) after 24 weeks of gestation.1 However, the odds ratio of fetal death associated with hypertension increased between the two study periods from 1.37 to 1.52.

A Canadian study of 1948 singleton pregnancies complicated with hypertension during 1986–1995 reported offspring death rates during the period 20 weeks of gestation until 30 days after birth. The death rate was 16/1000 in pregnancies with gestational hypertension, 32/1000 in chronic hypertension and 59/1000 in pre-eclampsia.26 An Italian study of 965 pregnancies with hypertension during 1986–1995 reported perinatal mortality (≥24 weeks of gestation until 7 days after birth) to be 12/1000 in pregnancies with gestational hypertension, 6/1000 in chronic hypertension and 26/1000 in pre-eclampsia.34 A Chinese population-based observational study among 16 000 pregnancies reported higher perinatal mortality in pregnancies with pre-eclampsia (17.8/1000) relative to pregnancies with gestational hypertension (10.2/1000).36 A review including 46 studies suggested that chronic hypertension tripled the risk of perinatal mortality relative to normotensive pregnancies.28

Moreover, the risk of fetal death seems to vary by type of hypertensive disease.24 One retrospective study from the USA, including more than 8000 pregnant women with hypertension, reported odds ratios of 2.9 for fetal death associated with chronic hypertension and 1.9 for fetal death associated with pregnancy-induced hypertension relative to normotensive women.16 A Swedish study of more than 1 million pregnancies during 1983–1992 reported fetal death rates after 28 weeks of gestation of 4.2/1000 in women with gestational hypertension, 5.3/1000 in women with pre-eclampsia and 11.2/1000 in women with chronic hypertension.25 In recent time, chronic hypertension, in particular, seems to be associated with a higher risk of fetal death.24

The RR of fetal death associated with hypertension is highest in term and post-term pregnancies.8,35 A study of more than 300 000 pregnancies reported an increase in the odds ratio of fetal death in women with chronic hypertension from 2.2 before 28 weeks of gestation to 3.3 at 28 weeks of gestation and beyond. The corresponding increase in women with pregnancy-induced hypertension was from 0.7 to 1.4, with normotensive women as the reference.8 In a study in the USA among 11 000 000 deliveries, the RRs of fetal death associated with pregnancy-induced hypertension were 1.2 at week 36 and 2.5 at week 41. In women with chronic hypertension, the RRs were 2.1 at week 36 and 4.4 at week 41, with low-risk pregnancies as reference.37 In a recent study of 171 000 women with chronic hypertension, the risk of fetal death increased from gestational week 38 (1.1 per 1000 ongoing pregnancies) to week 41 (3.5 per 1000).35

In our study, the risk of fetal death was increased in pregnancies with hypertension relative to normotensive women. The proportion of offspring that was stillborn was highest in early pregnancy, as intrauterine fetal death is an important reason for preterm induction and very preterm offspring may not survive birth because of immaturity. The patterns of fetal death risk associated with hypertension, however, were similar independent of the type of hypertension, with the highest RR of fetal death at and after term. These findings suggest that, for all types of maternal hypertensive disease, offspring are at increased risk of fetal death, and such pregnancies should be given similar clinical follow-up to ensure timely interventions.

In the time period 1987–2006, pregnancies with pre-eclampsia showed no increased risk of fetal death. In the Norwegian antenatal care programme, screening for pre-eclampsia is performed. In addition, women with pre-eclampsia may have more symptoms and may seek health care beyond the scheduled number of antenatal examinations.24 Thus, the reduction in fetal death in women with pre-eclampsia may therefore be attributed to the early detection of offspring at increased risk of death and to timely intervention in these pregnancies.14 The rate of premature deliveries (<37 weeks of gestation) increased significantly among women with pre-eclampsia during the study period (14.0% in 1967–1986 to 22.6% in 1987–2006, P < 0.001). This was also the case for women with gestational hypertension, whereas, for women with chronic hypertension, there was no change in the proportion that delivered prematurely (10.8 to 11.0%, P = 0.089). Despite the increased number of prematurely delivered infants among pre-eclamptic women, the perinatal mortality declined in the same manner as the fetal death rate; this has also been confirmed by earlier reports.14

In this registry study, we have no information on the use of anti-hypertensive treatment before or during pregnancy. The benefit of maternal anti-hypertensive therapy on the risk of fetal death is limited.38 Thus, the most efficient means to reduce fetal deaths in pregnancies with hypertension is to provide close clinical follow-up and induction of delivery in threatened pregnancies.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

In our nationwide study during 1967–2006, the risk of fetal death among women with hypertensive disorders in pregnancy has been greatly reduced, especially among pre-eclamptic women at term.

Disclosure of interests

There are no conflicts of interest.

Contribution to authorship

ASA analysed the data and wrote the manuscript. SOS contributed to data analysis, interpretation of the results and writing of the manuscript. Both authors approved the final version of the manuscript.

Details of ethics approval

The Medical Birth Registry of Norway is approved by the Norwegian Data Inspectorate. The Publishing Committee of the Medical Birth Registry approved our study.

Funding

The study was funded by the Norwegian Foundation for Health and Rehabilitation through the Norwegian SIDS and Stillbirth Society.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References

The authors acknowledge the Medical Birth Registry of Norway for providing data.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References