Pregnancy health

What’s the deal with blood clots during pregnancy?

The risk of blood clots from the AstraZeneca vaccine has called attention to the fact that women have always been at risk of blood clots from something else—pregnancy. Here’s what you need to know.

What’s the deal with blood clots during pregnancy?

Photo: iStock/burakkarademir

When Sandra Blitz was 22 weeks pregnant with her first child, she stood up from lying on the couch and was overcome by pain radiating down her leg, which was swollen and purplish. She phoned her husband, who was in surgical training at the time, and described her symptoms. He told her to go to hospital.

It turns out Blitz had developed a blood clot in a vein in her pelvis. The clot, called a deep vein thrombosis (DVT), in her groin, blocked blood from flowing into her left leg and she ended up staying in the hospital on blood thinners for two weeks. Though it was a frightening experience for the first-time mom, her main concern was for the health of her baby.

“Once I knew my baby was okay I was all right,” says the now mom-of-two in Kelowna, B.C. “The pain was substantial but when that started to subside I was fine.”

Blood clots during pregnancy are uncommon—there’s only a one in a thousand chance of developing one—but the even more rare risk of blood clots from the AstraZeneca vaccine has called attention to the fact that women can be at an increased risk of blood clots for many reasons throughout their lives.

Expecting mothers are at a higher risk of developing a blood clot during their nine months of pregnancy and also up to six weeks (and even 12 weeks in some cases) postpartum. Likewise, taking a combination birth control pill, with both estrogen and progestin, is associated with a risk of developing a blood clot about four times that of women not on a birth control pill, and that risk increases with age. (Higher doses of estrogen appear to increase clotting factors in the blood, which can lead to clot formation.)

Here’s what you need to know about these different types of blood clots.

Blood clots from the AstraZeneca vaccine

For unknown reasons, the AstraZeneca vaccine has triggered what hematologists call a “clotting storm”  in a small number of people who have received it. Essentially, an abnormal immune reaction has caused their platelets — the blood cells that help blood clot after an injury — to be activated and cause abnormal clotting.

“With this clotting storm you get these very serious clots that can occur in unusual places like the brain, liver, gut, or even in the arteries,” says Shannon Bates, director of the division of hematology and thromboembolism at McMaster University in Hamilton, Ont.


Right now, this looks to be a rare event, says Bates. While the rate is still being confirmed, The World Health Organization currently pegs the risk of developing a blood clot from the AstraZeneca vaccine at somewhere between four to eight in one million while a German study showed it to be 1 in 100,000.

Blood clots during pregnancy

In contrast, the women who develop a blood clot during pregnancy or postpartum do so for very different reasons.

For one, expectant mothers experience decreased blood flow during pregnancy because a growing baby puts pressure on the veins in the pelvis, restricting blood flow. In addition to other natural changes in the body that occur due to pregnancy, that’s why most blood clots during pregnancy occur in the legs (most commonly the left leg) and pelvis (though sometimes a clot will show up in a lung, likely because it travelled there from the leg).

Also, the body’s way of stopping bleeding, sometimes called its “clotting system”, gets activated during pregnancy, which makes the blood more prone to coagulation. The reason for this normal change is to stop the bleeding after delivery, which is a good thing.

Nobody really understands why the higher risk of developing a blood clot extends into postpartum—it could be that the body takes time to readjust to its clotting baseline, says Bates. The risk is highest immediately following delivery and drops as the weeks go by.


If you’re wondering why you didn’t hear much about blood clots in pregnancy previously, it’s because the majority of women will sail through pregnancy and postpartum without any clotting problems. However, there are a few factors that elevate the risk of developing a blood clot, including smoking, obesity and age (the older the woman, the greater the risk). Even more concerning is a personal or family history of blood clots; in this case, the family doctor, midwife or obstetrician would discuss options, such as starting on preventative blood thinners that are safe to take during pregnancy and while breastfeeding. When Sandra Blitz found out she was pregnant with her second child, her doctor immediately put her on blood thinners because she’d experienced a DVT during her first pregnancy. She had no clotting issues the second time around.

Pregnancy blood clot warning signs

With Blitz’s blood clot, it was fairly obvious that something was wrong — her leg was extremely tender, in addition to being swollen and purple — but not all clots show up the same way. Some of the symptoms, such as leg swelling, occur regularly during pregnancy. But normal swelling appears in both legs and is lessened by elevating the feet, whereas a clot typically causes only one leg to swell and it doesn’t get better by resting. Even shortness of breath or chest tightness, which can be a sign of a blood clot in the lungs, is common later in pregnancy from the additional weight women are carrying around.

Obviously, women shouldn’t rush to the ER every time their ankles swell or they get winded going up the stairs—the real red flag, says Bates, is when there are symptoms in combination.

“If the swelling is getting worse rapidly, or there’s swelling, pain and redness—that probably signifies something more worrisome,” says Bates. “The typical pain that people describe with a (lung clot) is a sharp pain that gets worse when you breathe in, or coughing up blood, or a racing heart, or feeling light-headed… They need to go to the emergency room right away—COVID or not.”

Fortunately, most patients respond well to the standard treatment of blood thinners once they arrive in hospital. And data suggest that less than 10 percent of all blood clots are fatal, says Bates. (This data does not include blood clots from the AstraZeneca vaccine.)

COVID vaccine and blood clots in pregnant people


Though it’s been 14 years since her blood clot, Blitz wonders if her history puts her at a higher risk of developing a vaccine-related clot. But pregnant women and women with a history of blood clots needn’t worry that they have an increased risk of blood clots from the vaccine, say experts. (There is one caveat, according to Bates. People who have had a brain clot or a HITT reaction (Heparin induced thrombosis) are being advised not to get the vaccine.)

“The blood clots that are being identified as potentially associated with the [AstraZeneca] vaccine are driven by an immune response to the vaccine that causes clotting,” says Chelsea Elwood, a reproductive infectious diseases specialist and OB/GYN at BC Women’s Hospital and Health Centre. “Those are fundamentally different from the blood clots we see in pregnancy, as well as after delivery. They are both clots, but the reason for them is very different.”

That means there’s no reason to avoid getting a vaccine if you’ve had a history of blood clots. “Right now we’re advising our patients with a prior history of blood clots that we don’t think they’re at higher risk for the AZ-related clotting,” says Bates. “Get whatever vaccine is first available to you.”

Likewise, the Society of Obstetricians and Gynaecologists of Canada (SOGC), supports the use of all COVID-19 vaccines approved in Canada in any trimester of pregnancy and during breastfeeding.

“We’re getting a lot of questions asking for clarity, and we’re pretty clear about the importance of vaccinating pregnant women in the current pandemic,” says Elwood, who is also the co-chair of the SOGC infectious diseases committee. “The best vaccine is any COVID-19 vaccine, and the best time to get it when you’re pregnant is any time.”

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