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Women who are thinking about getting pregnant or who become pregnant should know whether they are immune to chickenpox, new guidance from the Society of Obstetricians and Gynaecologists of Canada says.

Women who are thinking about trying to get pregnant and who aren't immune to the virus should be vaccinated at least four weeks before trying to conceive, it says. But women who are pregnant already should not be vaccinated during their pregnancy, because the vaccine is made with live – though weakened – viruses.

The guidance does not recommend the termination of a pregnancy if a woman is inadvertently vaccinated against chickenpox while she is pregnant. It notes a registry of 362 women who were vaccinated while pregnant reports no cases of congenital varicella (chickenpox) syndrome or other congenital malformations.

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The recommendations are part of guidance the country's obstetricians and gynecologists – ob-gyns for short – are giving to minimize the risk the chickenpox virus poses to pregnant women and the fetuses they carry.

"With chickenpox, timing is of the essence," Mark Yudin, one of the principal authors of the new guideline and chair of the society's infectious disease committee said in a statement.

"In order to provide the most effective and efficient care possible, it is important for health-care professionals to document a woman's history of previous chickenpox infection and varicella vaccination, ideally before conception or pregnancy."

It is estimated that only two to three of every 1,000 pregnant women exposed to chickenpox contract the varicella virus and develop complications. Based on that estimate, and the annual number of pregnancies in Canada – about 350,000 – 700 to 1,050 cases of chickenpox in pregnant women are expected to occur annually, the society says.

Infection with chickenpox always carries a small risk of death, and the risk is 15 times higher in adults than in children, the practice guideline notes. But the risk of a fatal infection is higher again in pregnant women, particularly women who are infected during the third trimester of pregnancy.

In addition, women who become infected during pregnancy are at risk of developing a condition call pneumonitis, which is inflammation of the lungs. It's estimated five to 10 per cent of pregnant women who contract chickenpox will develop pneumonitis – and some of them could require use of a mechanical ventilator to help with breathing.

Chickenpox infection during pregnancy can also harm fetuses, with infection crossing the placenta. Children born after their mothers were infected during the first half of pregnancy can have congenital malformations or deformations, including partial limb reduction.

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Other deformities can include chest wall malformations, hydrocephalus (water on the brain resulting in an enlarged head) and microcephaly, an abnormally small head.

It's estimated that over 90 per cent of Canadians are immune to chickenpox, either due to previous infection or vaccination. That means there aren't as many cases seen as there were decades ago, so pregnant women will be at lower risk of encountering the virus.

But those who don't know their immune status and are exposed to someone for an hour or more who has chickenpox should inform their health-care provider, the recommendations say.

Women in that situation should have their blood tested for antibodies, the society's guideline suggests.

And women who develop a severe chickenpox infection during pregnancy should be treated with antiviral drugs such as acyclovir.

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