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IVF linked to an increased risk of blood clots during pregnancy

By Kathy Oxtoby

Women who have in vitro fertilisation (IVF) have a higher risk of getting blood clots in the legs or the lungs during the first three months of pregnancy, a new study suggests.

What do we know already?

Many couples who have been unable to conceive turn to IVF in the hope that this treatment can help them have a child. For those women who do conceive after IVF there are risks, as with any pregnancy.

We already know that women have a higher risk of getting blood clots in the deep veins of the legs (venous thromboembolism, or VTE) during normal pregnancy. This affects around 1 in 1,000 pregnant women. Research shows that blood clots have been reported in more IVF pregnancies than normal pregnancies. But we don’t know how IVF affects the risk of pulmonary embolism (PE). Pulmonary embolism is when a blood clot blocks a blood vessel in the lungs. It can be fatal. Although uncommon, PE is one of the main causes of women dying while pregnant or giving birth.

Researchers compared the risk of both pulmonary embolism and blood clots in women undergoing an IVF pregnancy with women who were pregnant and had not had IVF.

Researchers looked at 23,498 women undergoing an IVF pregnancy and 116,960 women undergoing normal pregnancy during 1990 and 2008. The average age of women who took part in the study was 33, and all of them were pregnant for the first time. Researchers also took into account other risk factors such as weight, whether the women smoked, and their family circumstances.

What does the new study say?

During the study, the proportion of women having IVF who had blood clots in their legs was around 4 in 1,000, compared with 2.5 in 1,000 of pregnant women who had not had IVF. The risk was higher only during the first three months of pregnancy (1.5 in 1,000 of those who had IVF and 0.3 in 1,000 who did not). There was no difference in their risk of having blood clots in the legs before getting pregnant, or during the year after delivery.

The risk of having a blood clot in the lungs was also higher for women who had IVF, but the overall risk is still very low. Of the women in the study, 3 in 10,000 women who had IVF got a blood clot in the lungs, compared with fewer than 1 in 10,000 women who did not have IVF.

How reliable is the research?

This was a long term study involving a large number of women, which should mean its results are fairly reliable. However, this kind of study cannot prove that IVF treatment causes blood clots – only that there is a link between having IVF and the chances of developing PE and VTE.

What does this mean for me?

This study is a reminder that IVF is not a risk-free procedure. If you are planning IVF treatment, talk to your doctors about the risks of pregnancy and childbirth to you as an individual.

Ask your doctors about how these risks can be minimised. You can ask for advice about how to do this, which may include trying to be as healthy as possible before you start your treatment by eating a healthy diet, maintaining a healthy weight, and giving up smoking.


Henriksson P, Westerlund E, Brandt L, et al. Incidence of pulmonary and venous thromboembolism. BMJ. 2013; 346: e8632.

Jan 16, 2013