Researchers have said that pregnant women with chronic hypertension (high blood pressure) are highly likely to suffer from adverse pregnancy outcomes such as preterm delivery, low birth weight and neonatal death.

Chronic hypertension complicates between 1-5 per cent of pregnancies, and the problem may be increasing because of changes in the antenatal population.

Researchers from King's College London carried out a study to assess the strength of evidence linking chronic hypertension with poor pregnancy outcomes. They combined data from studies from 55 studies done in 25 countries.

The researchers looked at the following outcomes: preterm delivery (delivery before 37 weeks' gestation); low birth weight (below 2500g); perinatal death (fetal death after 20 weeks' gestation including stillbirth and neonatal death up to one month) and admission to neonatal intensive care or special care baby units.

The relative risk of pre-eclampsia (a condition in pregnancy characterised by high blood pressure) in women with chronic hypertension was on average nearly eight times higher than pre-eclampsia in non-hypertensive women. All adverse neonatal outcomes were at least twice as likely to occur, compared with the general population.

The researchers conclude that "chronic hypertension is associated with a high incidence of adverse pregnancy outcomes compared with a general population".

The study has been published in the British Medical journal.