Elizabeth Fraser jokes that she had one of the world’s most–planned pregnancies. Six months before she became pregnant, the midwife sat down with experts at The Ottawa Hospital to figure out how to reduce her risk of potentially dangerous blood clots.

Pregnant women are at higher risk of blood clots because their bodies are primed to stop the bleeding when they give birth. But Fraser also has a rare condition called essential thrombocythemia (ET) where her body produces too many of the cells that form blood clots.

To find out what she should do to reduce her risk of clots, she asked to see Dr. Marc Rodger, one of the world’s leading experts on blood clots in pregnancy.

“I’ve just had tremendous feedback from patients who have seen him, about how skilled and compassionate he is,” said Fraser.

However, Dr. Rodger didn’t know what to recommend because guidelines for pregnant women with ET did not exist. To solve this, Dr. Rodger’s colleague Dr. Leslie Skeith mined the scientific publications for data on more than 500 pregnant women with ET. The doctors’ meta–analysis, published in the journal Blood, showed that the risk of blood clots in this population was between 1.3 and 4.3 percent before birth, and between 1.2 and 9.5 percent after birth.

“We recommend that pregnant women who have a risk of blood clots over three percent should take blood thinners,” said Dr. Skeith, a thrombosis fellow and associate scientist at The Ottawa Hospital. “So in most cases, women with ET do not need to take blood thinners during pregnancy, but would likely need to take them after the baby is born. However, we recognize that some patients may choose to take blood thinners for their entire pregnancy.”

Dr. Skeith and Dr. Rodger calculated that Fraser’s risk of a blood clot during her pregnancy was between two and three percent.

“Because my risk level was in the middle, they basically said it was my choice,” said Fraser, who took blood thinners for her entire pregnancy and after the baby was born. “Getting information that was tailored to my experience was wonderful.”