Sophie King wanted a baby but no doctor knew what her chances were of making it through pregnancy alive.

The 31-year-old Owen Sound woman has a rare and life-threatening blood disease called paroxysmal nocturnal hemoglobinuria (PNH), which puts her at higher risk of blood clots, bone marrow failure and leukemia.

"It's so unknown," said King, who was diagnosed at 19 with the acquired disease in which red blood cells break down earlier than normal.

"When I got older, I started asking questions and there weren't any answers," she said. "Definitely one of the scariest things is knowing there is no information … They can't tell me what to expect."

Research involving McMaster University with "hard data" on case studies is now giving women with PNH reassurance that pregnancy no longer carries an up to one in five chance of death.

"It's no longer a discussion where you are trying to dissuade the mom," said Hamilton's Dr. Christopher Patriquin, an author of the study. "We are much more able to discuss the safety."

It's a far cry from when pregnancy was so dangerous that 8 to 20 per cent of moms with PNH died.

Only half of their babies made it to full term and as many as one in 10 died.

"Pregnancy was really discouraged and heavily cautioned against," said Patriquin, a clinical scholar in hematology and thromboembolism at McMaster. "All of the symptoms got worse … It was quite a significant health issue."

A drug called eculizumab, which blocks the breakdown of red blood cells, improved the quality of life and survival of PNH patients when it became available in 2007.

But women such as King had to take a blind leap when using it during pregnancy.

"I felt like a guinea pig," said King who asked for her pregnancy to be documented to help other women like her.

Until now, the only information came from four case reports, one small study involving seven pregnant women and another looking at two moms and babies.

"This is an extremely rare disease in a rare circumstance with very little data," said Dr. Ian Chin-Yee who is King's doctor and chief of hematology at the University of Western Ontario.

The patients "are aware of the unknowns and no place is it more frightening than in a mom worrying about herself and her baby," he said. "There are so few reports on it that it made the level of uncertainty that much greater."

The study published in the New England Journal of Medicine in September used the International PNH Registry to gather all known cases together for the first time and provide some evidence the drug is safe and effective for pregnant women.

"It formalizes the experience the doctors have had," said Barry Katsof, president of the Canadian Association of PNH Patients. Now women asking, 'Can I be a mother? Should I take the chance?' have some hard data."

None of the 61 women died during 75 pregnancies from June 2006 to November 2014. (Some were multiple births.)

And 69 of the babies lived. There was still an increased risk of prematurity, with nearly one-third of the babies delivered before 37 weeks of pregnancy.

Just over half of the moms had to take more frequent and higher doses of eculizumab during their pregnancies and they required more red blood cell transfusions. But none of them had blood clots during pregnancy and only two had them postpartum.

"They still are a high-risk pregnancy," said Chin-Yee. "It's still very anxiety-provoking for mom and baby."

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Among the biggest relief for moms is that the drug was not detected in breast milk. It was found in one-third of the cord-blood samples, but at a level that would not affect the baby.

No significant impairments were found in any of the children born in 31 hospitals in nine countries. More than 90 per cent of them had no developmental issues.

"It's definitely nice knowing there is more information on the condition," said King, who gave birth to Elizabeth, a healthy girl weighing five pounds 12 ounces on Nov. 15. "It makes you feel less lost."