Pregnant women may be more likely to suffer with the dangerous condition, pre-eclampsia, if they have asthma, a new study has found.

With pre-eclampsia, a woman's blood pressure can reach dangerously high levels and protein is present in her urine. The condition tends to occur during the second half of a pregnancy and it usually resolves once the baby is delivered.

Major problems arise if the condition develops early in the pregnancy, as decisions then have to be made on whether to deliver the baby prematurely.

Swedish researchers set out to examine the link between asthma in pregnancy and pregnancy/delivery outcomes. They studied data on more than one million births to over 700,000 women between 2001 and 2013. Some 10% of the babies born during this period had a mother with asthma.

The study found that women with asthma were more likely to suffer with pre-eclampsia.

"Four percent of all pregnant women develop pre-eclampsia, however we found that the risk of pre-eclampsia is 17% higher in women with asthma compared to women without asthma," noted the study's lead author, Dr Gustaf Rejnö, of the Karolinska Institutet.

The researchers also found that women with asthma were more likely to have shorter pregnancies, underweight babies, instrumental deliveries and caesarean sections.

These complications could not be explained by hereditary or environmental factors.

"It seems to be the asthma per se that causes these complications. This means that well-controlled asthma during pregnancy could reduce the relative incidence of complications during pregnancy and childbirth. In an earlier study, we saw that this was indeed the case," Dr Rejnö said.

Details of these findings are published in The Journal of Allergy and Clinical Immunology: In Practice.