For many women, pregnancy is a time of joy. But, for women with a history of blood clots, it can be a time of uncertainty.

Much like birth control, pregnancy increases a woman’s chance of experiencing a blood clot because her estrogen levels are higher than normal. Blood also becomes more prone to clotting during pregnancy, because the body is preparing to give birth.

“When you give birth, there’s bleeding. Your body is going to be a bit more inclined towards clotting, so that you can heal up and stop bleeding,” said Dr. Gregory Piazza, a cardiovascular medicine specialist at Brigham and Women’s Hospital. “We don’t want moms bleeding. It’s your body accounting for that.”

Dr. Piazza often works with women to help them manage their risk of clotting during pregnancy.

Women who have experienced a blood clot in the past, especially if it was related to an estrogen-based birth control, need to take certain precautions when pregnant or planning to become pregnant.

“If you’ve already had a blood clot due to high estrogen from the birth control pill, you’re going to need protection,” explained Dr. Piazza.

The Dangers of Blood Clots and Pregnancy

In addition to the known complications of blood clots, pregnant women can experience additional complications.

“Pregnant women are prone to developing pulmonary embolism and deep vein thrombosis, but they also can get pelvic vein thrombosis,” explained Dr. Piazza. “Pelvic veins are the veins that go to the legs but are higher up. Those can thrombose and cause massive leg swelling. Pregnant women can also get blood clots in the ovarian veins. These can cause abdominal, pelvic, or side pain. We try to protect patients from that.”

Blood clots can also affect the developing baby.

“Part of giving anticoagulation during the pregnancy is to achieve a live birth and prevent miscarriage,” Dr. Piazza remarked. “You can get blood clots in the umbilical cord or behind the placenta. Those can cause miscarriage.”

“It’s not something that’s talked about a lot for pregnant patients, so they tend to not really be aware of it,” said Dr. Piazza about the risk of miscarriage. “Even if they’re aware of the risk of blood clots, they’re more focused developing DVT or PE than the risk of miscarriage.”

Injectable Anticoagulants

How can pregnant women get the protection they need? From injectable anticoagulants.

“Usually what we do for pregnant patients is give them a low dose of an injectable anticoagulant, such as low molecular weight heparin or fondaparinux,” explained Dr. Piazza. “The good thing about those is that they tend not to affect the baby.”

“They’re large chains of molecules that tend not to cross over to the placenta. They’re safe to use,” Dr. Piazza continued. “We use them at preventive doses, not the full treatment dose. Patients do quite well on them and it’s very rare to see bleeding complications.”

Pregnant women with a risk of blood clots are taken off warfarin and novel oral anticoagulants, which have not been studied in pregnant women.

Post-Partum Risk

After giving birth, the post-partum period can be a risky time for mothers. This “post-partum risk period” can be anywhere from six to twelve weeks after the birth. Women are often told to continue their injectable anticoagulant medications.

“Those are safe for breast feeding moms. Those long molecular chains get digested by the baby’s stomach acid,” Dr. Piazza remarked, also noting that warfarin is appropriate for breastfeeding moms.

“Warfarin is great for breastfeeding moms because it doesn’t go into the milk. It’s one of the preferred treatments for breastfeeding moms.”

What Can Moms Do?

The most important thing for a concerned expectant mother can do is talk with her doctor and discuss her risk factors. If she has a personal or family history of blood clots, a genetic pre-disposition, or another risk factor, her doctor can help identify the best individualized medical treatment.

Beyond medical treatment, remaining active is key. Many women slow down when they are later into their pregnancy, but being sedentary increases the risk of developing a blood clot. Even light exercise, such as walking, can be very beneficial.

Finally, compression stockings, often used to help treat complications of deep vein thrombosis such as post-thrombotic syndrome, can help with the leg swelling that pregnant women often develop.

If you are a woman with a history of blood clots who is pregnant or planning to become pregnant, it’s important to speak with your doctor to decide what treatments and precautions are right for you.