Pregnant women with chronically high blood pressure are more likely to suffer complications such as early delivery of their babies, giving birth to newborns with a low birth weight and neonatal death, research suggests.
Chronic hypertension, or 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 cautioned that the figures will get higher because of a rise in the age of motherhood and high rates of obesity.
The researchers said that women with chronically high blood pressure should be targeted by health officials before they start trying for a baby to "optimise their health" before pregnancy.
In order to assess the link between chronic hypertension and poor pregnancy outcomes, the researchers from King's College London analysed data from 55 studies concerning almost 800,000 women.
They looked at pre-term delivery, when a mother gives birth before 37 weeks, a low birth weight below 2,500g, perinatal death - between 20 weeks gestation, including still birth, up to one month after birth - and admission to neonatal intensive care.
All of these adverse neonatal outcomes were at least twice as likely to occur, 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.
" Chronic hypertension is associated with a high incidence of adverse pregnancy outcomes compared with a general population," they wrote.
"Our results support the importance of increased antenatal surveillance for women with chronic hypertension to enable early identification of evolving complications.
"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."
Mervi Jokinen, practice and standards development advisor at the Royal College of Midwives, said: "This is an important issue highlighting the need for good antenatal care.
"Midwives are well aware of the risks of existing hypertension in pregnancy and the need for individualised care for each woman, supporting and advising women as well as providing clinical care.
"Maternity services have to ensure that midwives are able to work closely with different health care professionals in providing care for these women, therefore improving communication and pregnancy outcomes. These are more complex pregnancies requiring more time and resources and most importantly, enough midwives with enough time to support women so that they receive the best possible care and outcomes to their pregnancy."