Women who become pregnant as a result of IVF (in vitro fertilisation) have an increased risk of suffering from blood clots or artery blockage, particularly during their first trimester, a new study indicates.

With IVF, a woman's eggs are fertilised with sperm outside the body in a test tube - resulting in an embryo - and then placed back inside her womb. The first IVF baby was born in the UK in 1978 and since then, around five million children have been born worldwide using this treatment.

According to Swedish scientists, it is already well established that the risk of blood clots increases during a normal pregnancy.

While blood clots are generally reported more often in IVF pregnancies compared to normal pregnancies, to date, nothing is known about the risk of artery blockages in IVF pregnancies.

The scientists focused on venous thromboembolism (VTE), also known as blood clots, and pulmonary embolism (PE), which is a blockage in the main artery to the lungs.

They compared the risk of both of these conditions in over 23,000 women undergoing IVF and almost 117,000 women undergoing a normal pregnancy. The average age of the women was 33.

The study found that the proportion of women with a normal pregnancy who developed VTE was 2.5 per 1,000. However, in IVF women, this rose to 4.5 per 1,000. This risk was increased during the first trimester.

Meanwhile the risk of suffering PE was also slightly higher in women undergoing IVF. This risk remained during the entire pregnancy, but was especially apparent in the first trimester.

While the absolute risk of PE was low - just two to three additional cases per 100,000 IVF women - the scientists insisted that these findings are still important because the condition is difficult to diagnose and is still a leading cause of maternal death.

The results stood irrespective of other factors such as smoking, body mass index and age.

The scientists from the Karolinska Institute concluded that women who get pregnant via IVF have an increased risk of suffering an artery blockage. They added that all doctors should be aware of these findings as PE is a potentially deadly condition.

"Efforts should focus on the identification of women at risk," they added.

Details of these findings are published in the British Medical Journal.