Swedish researchers compared 23,498 women who had given birth after I.V.F. from 1990 to 2008 with 116,960 women of the same age and general health who had natural pregnancies. The results appeared online last week in the journal BMJ.
Women with I.V.F. pregnancies had more than four times the risk of venous thrombosis during the first trimester, compared with those with natural pregnancies, and almost seven times the risk of pulmonary embolism. The difference narrowed, but persisted, as the pregnancies progressed.
The I.V.F. procedure induces multiple egg production with high doses of hormones, and the authors suggest that this may be the cause.
"Women who are going to have I.V.F. should know these findings," said the lead author, Dr. Peter Henriksson, a professor of internal medicine at the Karolinska Institute. "And if they have had blood clots themselves, or have relatives with thrombosis, they should be treated with blood thinners."
Image: Pregnant woman in hospital, via Shutterstock