A new study has associated in vitro fertilization (IVF) with an increased risk for blood clots and pulmonary embolism (PE) in the first trimester of pregnancy. This is in addition to the already higher risk of blood clots seen during a normal pregnancy, which affected about one out of every 1,000 pregnancies in the early 90s.

IVF has been used since 1978 to help infertile couples conceive. Since that time, approximately five million people have been born using IVF.

The study, published this week in the British Medical Journal, looked at 23,498 women undergoing an IVF pregnancy and compared them to 116,960 women undergoing a normal pregnancy. Researchers found that 4.2 out of every 1,000 women in IVF pregnancies were diagnosed with blood clots, while only 2.5 in every 1,000 of the other women were. Though the risks were greater in the first trimester of pregnancy, there was no difference in risk between the groups of women either before pregnancy or during the year after pregnancy.

The researchers also found that the risks of PE were also higher for women in IVF pregnancies throughout their entire pregnancy. Though the overall risk for PE is low, it is the leading cause of maternal death and researchers stated that it is hard to diagnose.

The results of the study were controlled for a number of factors, including age, year of delivery, body mass index, smoking, birth country, and education. Researchers stated that the increased risks found in the study should help doctors focus on identifying women who are at greater risk.