We identified 210 814 first singleton births of babies weighing more than 200 g among women with no hypertension before 20 weeks' gestation and with no proteinuria, delivering between 24 and 43 weeks' gestation. The mean (SD) birth weight of the babies was 3282 g (545 g), and there were 1335 perinatal deaths. Recorded mean diastolic blood pressure at booking fell by an average of 0.23 mm Hg per year from 67.9 mm Hg in 1988 to 65.2 mm Hg in 2000. shows the mean highest diastolic blood pressures during pregnancy and perinatal death rates for selected variables. 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. Overall, there were 46 perinatal deaths in women excluded from the analysis with a history of hypertension and 58 perinatal deaths for those with proteinuria (including all cases of pre-eclampsia). In 10 of these women there was a history of hypertension and proteinuria; thus 94 perinatal deaths occurred in the excluded group; 93.4% of all perinatal deaths (1335 of 1429 deaths) therefore occurred in women without a history of hypertension and who did not develop pre-eclampsia.
Mean highest diastolic blood pressure in mothers during pregnancy and perinatal death rates for selected variables in cohort of 210 814 first singleton births in North West Thames region, London*
shows diastolic blood pressures at booking for antenatal checks and weeks of gestation among 169 249 women when both were recorded. Substantial numbers of women had their first antenatal check at all gestations from eight to 40 weeks, thus providing cross sectional data on blood pressures through pregnancy. Mean diastolic blood pressure was 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 or more. 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 also observed in the unadjusted data.
Before 34 weeks' gestation there was no substantial correlation between highest blood pressure in pregnancy and birth weight (data not shown). At 34 weeks' gestation or more, however, there was an inverted U shaped relation between birth weight and blood pressure, with a maximum birth weight at around 80 mm Hg (). When birth weight was adjusted for maternal height and weight, the relation remained similar but shifted slightly towards lower blood pressures. The relation of birth weight with blood pressure could not be explained by confounding by gestational age; analysis of the Z scores of birth weights (by gestational age) showed the same relation (data not shown). To account for the effects of obstetric interventions on relation between highest antenatal blood pressure and birth weight, 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 (data not shown).
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.
Fig 3 Birth weight and rise in blood pressure after booking for antenatal checks, by blood pressure at booking and gestation between 34 and 43 weeks at delivery, among women without chronic hypertension and no proteinuria. Values adjusted for calendar year (more ...)
Perinatal mortality showed a strong curvilinear association with highest diastolic blood pressure (). 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 (). 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 even when we included women with proteinuria or chronic hypertension (); in linear quadratic models, the quadratic term was still highly statistically significantly different from zero. In this larger group, 98.7 perinatal deaths (11.2%) were attributable to mothers with blood pressure differing from the optimal blood pressure (80.4 mm Hg) predicted by the linear quadratic model; most (70.1%) of these excess perinatal deaths (69.2) occurred among mothers with lower blood pressures (see ).
Number of perinatal deaths, by blood pressure range, attributed to mother's highest blood pressure during antenatal check being more or less than optimal value predicted by linear quadratic model