PREGNANT women with chronically high blood pressure are more likely to have complications such as pre-term delivery, giving birth to newborns with a low birth weight and neonatal death, research suggests.
Chronic hypertension, or high blood pressure, could also lead to a higher risk of pre-eclampsia and Caesarean sections, the study found.
The condition complicates between 1% and 5% of pregnancies, but the study authors said the figures will only get higher because of the rising age of mothers and high rates of obesity.
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The researchers said women with chronically high blood pressure should be targeted by health workers before they start trying for a baby to "optimise their health" before pregnancy.
The researchers, from King's College London, analysed data from 55 studies covering almost 800,000 women. They looked at pre-term delivery, when a mother gives birth before 37 weeks, a low birth weight below 2500g, perinatal death -between 20 weeks gestation, including stillbirth, up to one month after birth - and admission to neonatal intensive care.
All of these adverse outcomes were at least twice as likely in women with chronic hypertension when compared with the general population, they found.
And they estimated that 29% of women with chronic hypertension will have pre-eclampsia and 42% would have a c-section.
They wrote: "Our results support the importance of increased antenatal surveillance for women with chronic hypertension.
"Women should receive pre-pregnancy counselling to optimise their health before pregnancy and to inform them of the increased maternal and fetal risks associated with their hypertension."