Women who have suffered a stillbirth or who have certain conditions, including varicose veins or inflammatory bowel disease (IBD), may be at an increased risk of developing dangerous blood clots after giving birth, a new study indicates.

Scientists in the UK looked at risks linked to VTE (venous thromboembolism). This is a collective term for deep vein thrombosis (DVT) and pulmonary embolism (PE).

DVT is a condition where a blood clot forms within the deep vein system, often in the lower part of the body such as the legs. The main complication of DVT is PE, which is the name given to a blood clot that lodges in one of the blood vessels that supplies the lungs with blood.

Up to one in four patients with VTE die within a week and almost one in three survivors experience long-term effects.

"VTE is a rare but serious complication of pregnancy and childbirth. It affects around one or two pregnancies per 1,000 but, despite this, remains a leading cause of mortality in expectant and new mothers in developed countries," explained lead scientist, Dr Matthew Grainge, of the University of Nottingham.

The study assessed data relating to almost 400,000 pregnancies that had taken place between 1995 and 2009. The mothers were aged between 15 and 44.

All women who developed VTE for the first time during their pregnancy were identified and then other health information was assessed, such as whether they smoked and what their body mass index (BMI) was. Pregnancy-related issues were also taken into account, such as complications and how the baby was delivered.

The study found that women who had suffered a stillbirth were more at risk of developing VTE after the pregnancy. Women were also more at risk after giving birth if they had varicose veins, IBD, heart disease or were obese.

Women who suffered bleeding during their pregnancy or labour, those who gave birth prematurely and those who required a Caesarean section also had an increased risk of developing VTE after giving birth.

However the risk of suffering VTE during the actual pregnancy was only slightly higher if the woman was over the age of 35, was overweight or obese or was a smoker.

Overall, the risks were greater among new mothers. For example, obese women were four times more likely to develop VTE, while having a baby by Caesarean section, bleeding in pregnancy and giving birth prematurely doubled the risk in new mums.

The biggest risk appeared to be among those who had suffered a stillbirth - they were six times more likely to develop VTE.

"Our results may have important implications for the way that preventative measures for VTE are delivered in healthcare settings in developed countries. We believe the strong association between stillbirths and premature births and VTE in particular is a finding of real importance, which has received only limited attention to date.

"They are not currently incorporated in the guidelines for risk assessment for VTE and, if they were, then many cases associated with those risk factors could potentially be prevented." Dr Graine explained.

Details of these findings are published in the journal, Blood.

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