Published: 17-01-2013 00:00 | Updated: 10-06-2015 10:25

Blood clots more likely during IVF pregnancies

In vitro fertilization (IVF) is associated with an increased risk of pulmonary embolism (blockage of the main artery of the lung) and venous thromboembolism (blood clots) during the first trimester of pregnancy, according to a novel study by scientists at Karolinska Institutet. The study is published online in the British Medical Journal (BMJ).

It is well known that the risk of blood clots is increased during normal pregnancy. Blood clots have also been reported in more IVF pregnancies than normal pregnancies, but so far there is no information on the risk of artery blockage following IVF, which is important since this is a leading case of maternal deaths.

In their study researchers at Karolinska Institutet compared the risk of both pulmonary embolism (PE) and venous thromboembolism (VTE) in 23,498 women undergoing an IVF pregnancy and 116,960 women undergoing normal pregnancy. Women were matched for age and time period (births between 1990 and 2008). The women had an average age of 33 for both groups.

The proportion of IVF women who were diagnosed with VTE was 4.2 in 1000 compared with 2.5 in 1000 of the unexposed women. Risk was increased during the first trimester (1.5 in exposed and 0.3 in unexposed). There was no difference in risk prior to pregnancy or during the year after delivery. Researchers also identified 19 women with PE in the IVF group (0.08%) compared with 70 women in the unexposed group (0.05%). The risk of PE in the IVF women was increased during the whole pregnancy and particularly in the first trimester (a sevenfold increase). Results were not affected when adjustments were made for: maternal age, calendar years of delivery, BMI, parity, smoking, country of birth, family situation and education.

Absolute risks for PE were low however with 2-3 additional cases per 10,000 IVF women. However, because PE is a difficult condition to diagnose and still represents a leading cause of maternal death, the researchers recommend physicians should focus on the identification of women at risk.

Approximately five million individuals have so far been born after IVF. The current study was funded by the Swedish Research Council and through ALF, which is a regional agreement on medical training and clinical research between Stockholm County Council and Karolinska Institutet.

Much of this article was originally published as a press release from the BMJ.


Incidence of pulmonary and venous thromboembolism in pregnancies after in vitro fertilisation: cross sectional study.
Henriksson P, Westerlund E, Wallén H, Brandt L, Hovatta O, Ekbom A
BMJ 2013 Jan;346():e8632