One in every 20 cases of the serious condition of pregnancy, pre-eclampsia, may be linked to increased levels of the air pollutant ozone during the first three months, suggests a large study published in the online journal BMJ Open.
Mothers with asthma may be more vulnerable, the findings indicate.
Pre-eclampsia is characterised by raised blood pressure and the presence of protein in the urine during pregnancy. It can cause serious complications, if left untreated.