High blood pressure linked to adverse pregnancy outcomes

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, which emphasises a need for heightened surveillance, according to research carried out at King's College London and published today in British Medical Journal .

Chronic hypertension complicates between 1-5 per cent of pregnancies and recent demographic changes in the antenatal population suggest that chronic hypertension in pregnancy may be an increasing clinical problem.

A recent study in the US suggests the prevalence of chronic hypertension increased from 1995-1996 to 2007-2008, after adjustment for maternal age. Obesity and metabolism are likely to contribute and therefore the number of women entering pregnancy with chronic hypertension is set to rise.

There is an absence of strong evidence linking chronic hypertension with poor pregnancy outcomes so researchers from King's carried out a study to assess this risk.

They took data from studies on the general US population of pregnant women in 2006 using Medline, Embase and Web of Science and data from another 55 studies in total, from 25 countries.

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

The relative risk of pre-eclampsia in women with chronic hypertension was on average nearly eight times higher than pre-eclampsia in the general population. All adverse neonatal outcomes were at least twice as likely to occur, compared with the general population.

The researchers say this meta-analysis confirms that chronic hypertension is associated with adverse pregnancy outcomes. Across study populations, rates of pre-eclampsia, caesarean section, preterm delivery, low birth weight, perinatal death and neonatal unit admissions were all significantly higher for women with chronic hypertension than in general US pregnancy populations.

The researchers conclude that 'chronic hypertension is associated with a high incidence of adverse pregnancy outcomes compared with a general population'. They stress the importance of increased antenatal surveillance for women with chronic hypertension and suggest they should receive pre-pregnancy counselling to optimise their health prior to pregnancy. They also say that strategies to predict those at greatest risk are needed.

Dr Lucy Chappell, Clinical Senior Lecturer in Maternal and Fetal Medicine, said: 'Having examined papers reporting on nearly 800,000 pregnancies across the world, our study is the largest to demonstrate a strong link between women with pre-existing high blood pressure in pregnancy and several adverse outcomes. There is an urgent need for heightened surveillance of women with chronic hypertension, which could include regular antenatal visits, home monitoring and ultrasound screening programmes.

'Greater awareness of this issue is crucial as the responsibility for monitoring chronic hypertension lies with all medical practitioners - not just obstetricians.'

Dr Chappell added: 'The problem is that we still do not know the most effective, safe and cost-effective way to manage chronic hypertension during pregnancy, or which interventions will have a real impact on these adverse outcomes. Further research is needed to establish best clinical practice and thus improve the outlook for mothers and their babies.'