BMJ  2004;329:1312 (4 December), doi:10.1136/bmj.38258.566262.7C (published 23 November 2004)

Paper

Maternal blood pressure in pregnancy, birth weight, and perinatal mortality in first births: prospective study

¹ Academic Department of Obstetrics and Gynaecology, Imperial College London, Faculty of Medicine, Chelsea and Westminster Hospital, London SW10 9NH, ² Department of Epidemiology and Public Health, Imperial College London, Faculty of Medicine, London W2 1PG

Abstract

Objective To investigate the relation of diastolic blood pressure in pregnancy with birth weight and perinatal mortality.

Design Prospective study.

Setting 15 maternity units in one London health region, 1988-2000.

Participants 210 814 first singleton births of babies weighing more than 200 g among mothers with no hypertension before 20 weeks' gestation and without proteinuria, delivering between 24 and 43 weeks' gestation.

Main outcome measures Birth weight and perinatal mortality.

Results The mean (SD) birth weight of babies born to mothers with no hypertension before 20 weeks' gestation or proteinuria was 3282 g (545 g) and there were 1335 perinatal deaths, compared with 94 perinatal deaths among women with proteinuria or a history of hypertension. Diastolic blood pressure at booking for antenatal checks was progressively higher from weeks 34 to 40 of gestation. The birth weight of babies being delivered after 34 weeks was highest for highest recorded maternal diastolic blood pressures of between 70 and 80 mm Hg and lower for blood pressures outside this range. Both low and high diastolic blood pressures were associated with statistically significantly higher perinatal mortality. Using a linear quadratic model, 94 of 825 (11.4%) perinatal deaths could be attributed to mothers having blood pressure differing from the optimal blood pressure (82.7 mm Hg) predicted by the fitted model. Most of these excess deaths occurred with blood pressures below the optimal value.

Conclusion Both low and high diastolic blood pressures in women during pregnancy are associated with small babies and high perinatal mortality.

Introduction

Few studies have reported fetal outcomes at lower maternal blood pressures. Low blood pressure is particularly common in underweight women with a low muscle mass. We hypothesised that the association of a low maternal weight with an increased risk of a low birthweight baby and poor infant survival¹ might be due in part to low blood pressure leading to poor placental perfusion. Three small studies reported that low maternal blood pressure in pregnancy is associated with low birth weight and an increased risk of small for gestational age babies.^2-4 Another study reported an increased risk of perinatal mortality.⁵ Since 1988, prospective data have been collected on all pregnancies booked into 15 of the 17 maternity units in the North West Thames region of London using the St Mary's Maternity Information System. We used this database to investigate the association between blood pressure in pregnancy and fetal outcome (birth weight and perinatal mortality) in women without pre-existing hypertension or proteinuria.

Materials and methods

From 1988-2000, the St Mary's database included 517 381 entries with a recorded birth weight. Data were entered by trained staff, with online validation, prompting, and standard definitions for clinical measurements.⁶ We restricted our study to singleton babies born to nulliparous women (ensuring that only one delivery was included for each woman) between 24 and 43 weeks' gestation and weighing more than 200 g. For most analyses we excluded births to women with pre-existing (chronic) hypertension or with persistent proteinuria at any stage of pregnancy. This left data on 210 814 mothers and their births.

Blood pressure was recorded during the first and subsequent antenatal checks. In 1988, clinicians mostly recorded diastolic blood pressure using the Korotkoff phase IV heart sound. Since 1988 there has been a gradual change to using the Korotkoff phase V heart sound.

When women delivered, the midwives determined the diastolic blood pressure and gestational age recorded at the first antenatal check and the highest diastolic blood pressure during pregnancy.

We defined perinatal mortality as a stillbirth (death before birth at, or after, 28 weeks' gestation until October 1992, and 24 weeks thereafter) or the death of a baby in its first week of life. We cross checked deaths against national death registrations and data from the Confidential Enquiry into Stillbirths and Death in Infancy.⁷

Along with data on gestational age at booking for antenatal checks and birth and persistent important proteinuria, we recorded the mothers' ethnic group, smoking status, height, weight, age at booking, and any major medical disorders.

Statistical analysis
We calculated the body mass index of each woman and her Carstairs' deprivation score.⁸ For diastolic blood pressure and birth weight we adjusted for mother's ethnic group, smoking status, age at booking for antenatal checks, Carstairs' score, and height and weight. We adjusted blood pressure measurements for calendar year because of the secular shift from the Korotkoff phase IV to phase V heart sound.

We fitted log linear models to explore the relations between diastolic blood pressure and gestational age at booking and between highest maternal diastolic blood pressure and birth weight; logistic models were used to model perinatal mortality in relation to these variables (see bmj.com).

Results

The mean (SD) birth weight of the babies was 3282 g (545 g). Overall there were 1335 perinatal deaths in women without a history of hypertension and who did not develop pre-eclampsia, and 94 perinatal deaths among those with either chronic hypertension or proteinuria. We found statistically significant differences between groups for blood pressure by age at booking for antenatal checks; weeks of gestation at booking; mother's body mass index, ethnic group, Carstairs' score, and smoking status; birth weight; and year of birth. We also found statistically significant differences between groups for perinatal mortality for all variables except year of birth (see bmj.com).

Analysis of diastolic blood pressures at booking for antenatal checks and weeks of gestation among 169 249 women when both were recorded showed a mean diastolic blood pressure of 66.6 mm Hg in the first trimester and 66.3 mm Hg in the second trimester. It was progressively higher after 34 weeks' gestation, reaching 68.4 mm Hg by 40 weeks' gestation. This was after adjustment for the mother's ethnic group, smoking status, height and weight, calendar year, age at booking, and Carstairs' score: similar changes were observed in the unadjusted data.

At 34 weeks' gestation or more, there was an inverted U shaped relation between birth weight and blood pressure, with a maximum birth weight at around 80 mm Hg (fig 1). When birth weight was adjusted for maternal height and weight, the relation remained similar. The relation of birth weight with blood pressure could not be explained by confounding by gestational age. When we reanalysed the data only for women with spontaneous onset of labour at term (37-42 weeks' gestation), we found no material change in the results.

View larger version (35K): [in this window] [in a new window]   Fig 1 Birth weight (95% confidence intervals) and highest diastolic blood pressure, unadjusted or adjusted for maternal height and weight, among mothers without chronic hypertension, chronic renal disease, or proteinuria and gestation between 34 and 43 weeks at delivery. Values adjusted for calendar year and women's ethnic group, smoking status, height, weight, age at booking, and Carstairs' deprivation score

 

Birth weight in relation to rises in blood pressure during pregnancy depended on the blood pressure at booking. A blood pressure of 70 mm Hg or more at booking was associated with the highest birth weights as long as the blood pressure rise during pregnancy did not exceed 10 mm Hg; with rises greater than this, birth weight fell sharply. If the blood pressure at booking was less than 70 mm Hg, birth weight rose as the rise became greater, but started to fall again if the rise exceeded 30 mm Hg.

Perinatal mortality showed a strong curvilinear association with highest diastolic blood pressure (fig 2). In particular, if linear quadratic models were fitted to the data, the quadratic term was highly statistically significantly different from zero and remained so even when we excluded women with highest diastolic pressures of less than 60 mm Hg. This relation largely disappeared if corrected for birth weight (fig 2). Of the 824 perinatal deaths contributing to this analysis, we estimated that 94.3 (11.4%) were attributable to women with blood pressure differing from the optimal blood pressure (82.7 mm Hg) predicted by the linear quadratic model; most (91.2%) of these excess perinatal deaths (86.1) occurred among women with lower blood pressures, mainly in the ranges 70-79 mm Hg (51.2 excess deaths) and 60-69 mm Hg (30.9 excess deaths). Perinatal mortality still exhibited a strong curvilinear association with highest diastolic blood pressure when we included women with proteinuria or chronic hypertension (see bmj.com).

View larger version (41K): [in this window] [in a new window]   Fig 2 Perinatal mortality per 1000 live births and stillbirths (95% confidence intervals) in relation to highest diastolic blood pressure in mothers and gestation between 24 and 43 weeks at delivery, unadjusted and adjusted for birth weight, among women without chronic hypertension and no proteinuria (numbers of deaths are in brackets). Values adjusted for calendar year and women's ethnic group, smoking status, height, weight, age at booking, and Carstairs' deprivation score

 

Discussion

During pregnancy, both low and high diastolic blood pressures in women are associated with small for gestational age babies and high perinatal mortality. Blood pressure in women without pre-existing hypertension or pre-eclampsia falls slightly during the first half of pregnancy, but then rises from around 34 weeks onwards, suggesting a physiological mechanism.^9-12 Our analysis was based on a single blood pressure measurement for women at booking for antenatal care. The findings are similar to longitudinal observations on women followed through pregnancy, but with smaller datasets.¹³ We adjusted for multiple potential confounders, since booking late for antenatal care is associated with adverse socioeconomic factors that might influence blood pressure.

What is already known on this topic The measurement of maternal blood pressure is a key part of antenatal care Hypertension is associated with low birth weight and increased perinatal mortality, but data on the effects on the fetus of low maternal blood pressure are scarce What this study adds Birth weight is maximal and perinatal mortality lowest when the highest maternal diastolic blood pressure during pregnancy is between 70 and 90 mm Hg Low as well as high blood pressures is associated with low birth weight and increased perinatal mortality.

From 34 weeks' gestation onwards, birth weight was maximal when the highest recorded blood pressure during pregnancy was between 70 and 90 mm Hg diastolic. The relation was an inverted U shape, so that both higher and lower blood pressures were associated with lower birth weights.

A positive quadratic (U shaped) association was also seen with perinatal mortality, largely mediated through the association of blood pressure with birth weight. The size of our database confers high statistical power. We included over 1300 perinatal deaths, which is five and 14 times larger than the two largest previous studies.^{5 14}

Our findings support two previous small studies of women with low blood pressure in pregnancy that found lower birth weights and an increase in preterm deliveries⁴ and small for gestational age babies.² In another study, birth weight was related to low blood pressure as well as high blood pressure in the third trimester.⁵ In a controlled trial to prevent pre-eclampsia, atenolol produced a significant reduction in the incidence of pre-eclampsia but also reduced mean birth weight¹⁵; in a meta-analysis of antihypertensives in pregnancy, a mean diastolic blood pressure lower by 10 mm Hg or more was associated with an increased risk of small for gestational age babies.¹⁶

A high blood pressure at booking is advantageous for birth weight as long as there is only a small rise (< 15 mm Hg) in blood pressure thereafter. If the blood pressure at booking is low (70 mm Hg or less), however, a rise of 15-30 mm Hg is beneficial. This supports the concept that the rise in blood pressure in the third trimester is advantageous for fetal growth and may be a response to placental function failing to keep pace with fetal growth.

Strengths of our study include the large number of women studied and their geographical coherence. A potential weakness is that the data were collected as part of routine obstetric practice, with the variability that implies. Women with high blood pressure in pregnancy are more likely to have received interventions such as early induction of labour, thereby reducing the incidence of high blood pressure and increasing the risk of low birthweight babies. This would overestimate the effect of high blood pressure on low birth weight. Separate analysis of data from women with spontaneous onset of labour at term, however, gave essentially unchanged results. Women with low blood pressure were more likely to have booked late for antenatal care and therefore to have fewer antenatal checks, but the differences were slight.

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This is the abridged version of an article that was posted on bmj.com on 23 November 2004: http://bmj.com/cgi/doi/10.1136/bmj.38258.566262.7C

We thank the midwives who entered the data and the supportive clinicians and data guardians at the participating maternity units.

Contributors: See bmj.com

Funding: None.

Competing interests: None declared.

Ethical approval: This study was approved by the St Mary's NHS Trust local research ethics committee.

References

1. Cogswell ME, Yip R. The influence of fetal and maternal factors on the distribution of birthweight. Sem Perinatol 1995;19: 222-40.[CrossRef]
2. Grunberger W, Leodolter S, Parschalk O. Maternal hypotension: fetal outcome in treated and untreated cases. Gynecologic Obstetric Invest 1979;10: 32-8.
3. Margulies M, Voto LS, Fescina R, Lastra L, Lapidus AM, Schwarcz R. Arterial blood pressure standards during normal pregnancy and their relation with mother-fetus variables. Am J Obstet Gynecol 1987;156: 1105-9.[Web of Science][Medline]
4. Ng PH, Walters WA. The effects of chronic maternal hypotension during pregnancy. Aust NZ J Obstet Gynaecol 1992;32: 14-6.[CrossRef][Web of Science][Medline]
5. Friedman EA, Neff RK. Hypertension-hypotension in pregnancy. Correlation with fetal outcome. JAMA 1978;239: 2249-51.[Abstract/Free Full Text]
6. Cleary R, Beard RW, Coles J, Devlin HB, Hopkins A, Roberts S, et al. The quality of routinely collected maternity data. Br J Obstet Gynaecol 1994;101: 1042-7.[Web of Science][Medline]
7. CESDI. Confidential enquiry into stillbirths and deaths in infancy. London: Maternal and Child Health Consortium, 1998. (5th Annual Report.)
8. Carstairs V, Morris R. Deprivation and health in Scotland. Aberdeen: Aberdeen University Press, 1991.
9. Reiss RE, O'Shaughnessy RW, Quilligan TJ, Zuspan FP. Retrospective comparison of blood pressure course during preeclamptic and matched control pregnancies. Am J Obstet Gynecol 1987;156: 894-8.[Web of Science][Medline]
10. Clapp JF, Seaward BL, Sleamaker RH, Hiser J. Maternal physiologic adaptations to early human pregnancy. Am J Obstet Gynecol 1988;159: 1456-60.[Web of Science][Medline]
11. Halligan A, O'Brien E, O'Malley K, Mee F, Atkins N, Conroy R, et al. Twenty-four-hour ambulatory blood pressure measurement in a primigravid population. J Hypertens 1993;11: 869-73.[CrossRef][Web of Science][Medline]
12. Brown MA, Robinson A, Bowyer L, Buddle ML, Martin A, Hargood JL, et al. Ambulatory blood pressure monitoring in pregnancy: what is normal? Am J Obstet Gynecol 1998;178: 836-42.[CrossRef][Web of Science][Medline]
13. Moutquin JM, Rainville C, Giroux L, Raynauld P, Amyot G, Bilodeau R, et al. A prospective study of blood pressure in pregnancy: prediction of preeclampsia. Am J Obstet Gynecol 1985;151: 191-6.[Web of Science][Medline]
14. Page EW, Christianson R. The impact of mean arterial pressure in the middle trimester upon the outcome of pregnancy. Am J Obstet Gynecol 1976;125: 740-6.[Web of Science][Medline]
15. Easterling TR, Brateng D, Schmucker B, Brown Z, Millard SP. Prevention of preeclampsia: a randomized trial of atenolol in hyperdynamic patients before onset of hypertension. Obstet Gynecol 1999;93: 725-33.[CrossRef][Web of Science][Medline]
16. Von Dadelszen P, Ornstein MP, Bull SB, Logan AG, Koren G, Magee LA. Fall in mean arterial pressure and fetal growth restriction in pregnancy hypertension: a meta-analysis. Lancet 2000;355: 87-92.[CrossRef][Web of Science][Medline]

(Accepted 14 September 2004)

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