Your Sex and Hormones Impact Your Risk for Deep Vein Thrombosis

Deep vein thrombosis (DVT) is a circulatory system disorder that occurs when a blood clot forms in a deep vein, usually in the leg or pelvis. Symptoms of DVT can include pain, redness or swelling in the affected area. Sometimes, there are no symptoms.

A DVT clot becomes dangerous—it can be life-threatening—when the clot dislodges and travels to the lungs. This is called a pulmonary embolism (PE). Men are more likely to develop DVT and PE than women, but there are certain times in a woman's life when she is particularly vulnerable.

Understanding DVT

"To understand deep vein thrombosis, you need to understand something called Virchow's triad," said Christopher Hollingsworth, M.D., a general and endovascular surgeon in New York City.

Virchow's triad maintains that clots form for three basic reasons, Hollingsworth said, including the following:

• Slow, stagnant or disrupted blood flow
• Injury to the lining of the veins
• Abnormally sticky blood that clots too easily

"Most people that form clots have more than one risk in two or three of these categories," Hollingsworth said. "You have to think about DVT in the context of multiple risk factors coming together to form a perfect storm."

Oral contraception and DVT

About 21 percent of American women of childbearing age, from ages 18 to 49, used a birth control pill in 2018, according to the Guttmacher Institute. Those pills fall under two main classifications:

• Progestin-only pills, a synthetic form of progesterone
• A combination pill that contains progestin and estrogen

Combination oral contraception (COC) may increase the risk of blood clots. For women with no other risk factors, this is not a concern. The increase in risk from COC is lower than that caused by pregnancy.

If a woman has other risk factors, such as being overweight, the risk of DVT needs to be considered when determining which contraception to use.

Compared to non-overweight women who did not take a combination oral contraceptive, overweight women who took a COC had a 12-fold increase in developing DVT. Obese women's risk increased 24-fold, according to a large 2022 study.

The risk from obesity had the largest impact on women younger than 40. The greatest risk of clots occurs during the first year of COC use.

Since progestin-only pills do not increase the risk of DVT, they are usually recommended for women with obesity.

Pregnancy and DVT

Pregnancy increases the risk of DVT, and the risk carries into the postpartum period, where women have a 60-fold increased risk in the few months following delivery, a 2008 study suggested.

"First, in preparation for the potential blood loss that may occur at delivery, the blood of a pregnant woman can become thicker and will tend to clot easier than a non-pregnant woman's to minimize blood loss during delivery," said Collin Johnston, D.O., a board-certified physician who specializes in vascular health with Vein Envy in the Phoenix area.

"Additionally, toward the end of the pregnancy, the fetus may be putting unequal pressure on some important veins in the pelvic region, which can lead to decreased circulation down the legs and can potentially lead to an environment that is more conducive to forming a deep vein thrombosis," Johnston said.

Most women who get blood clots during pregnancy have several other risk factors, according to Hollingsworth, including the following:

• Varicose veins
• Multiple births
• Obesity
• Diabetes
• Increased age
• C-section delivery
• High blood pressure
• Cigarette use
• Eclampsia

Using Virchow's triad, the risk factors in each of the three categories common in pregnancy, according to Hollingsworth, include the following:

• Venous stasis (stagnation in the veins). A woman's blood volume increases by about 50 percent by the end of pregnancy, a 2016 report suggested. To accommodate this extra fluid, hormones cause the mother's veins to dilate or become larger. Swollen ankles are a common occurrence during pregnancy, caused by extra fluid pooling in the lower legs thanks to gravity. DVT risk increases with the hormonal surge that causes the dilation of veins and slows blood flow.
• Blood vessel injury. Labor causes major injury to the blood vessels of the pelvis.
• Hypercoagulability (increased likelihood of clotting). Pregnancy causes the blood to become stickier to limit the amount of blood loss during delivery.

"It's amazing all pregnant women don't get DTVs," Hollingsworth said. "Overall, blood clots in pregnancy are still rare: 1 in 500 to 2,000 pregnancies."

Treatment of DVT during pregnancy

DVT can be treated during pregnancy, but the options are limited.

"Not pregnant, you take an oral anticoagulant for a few months and then have a procedure to break up the DVT," Hollingsworth said. "But pregnant women have to stick themselves with a needle two or three times a day for the rest of the pregnancy. And since they can't expose the fetus to radiation, most modern interventions to break up a clot are off the table, because they are all guided by X-ray."

The challenge of treating clots during pregnancy makes it all that more important to have a lifestyle that decreases the odds of developing a clot. Lifestyle modifications may include regular physical activity, not smoking and wearing graduated compression socks.

How does hormone replacement therapy affect DVT risk?

Hormone replacement for menopause led to a two‐fold higher risk for DVT compared to nonusers in a large-scale 2004 study. Older age, increased body mass index and genetic clotting disorders further increased the risk.

Hormone therapy is a topic of intense study right now. In time, it will become clearer which formulation and delivery of which hormones cause the greatest risk to women.

DVT risk for men versus women

Over a lifetime, men have a greater risk of DVTs compared to women, a 2022 study suggested. Doctors do not understand why.

"Men are typically larger than women, and there are a number of studies that show body height is an independent risk factor for DVT," Hollingsworth said. "It is thought that the larger leg veins are more prone to venous stasis."

Body size is thought to explain about 20 percent of the difference in male versus female rates, a 2014 study suggested.

Breaking down the first DVT event by age group shows an interesting trend, the researchers found.

Women up to age 60 are more likely to have a DVT than men of the same age. This increase seems to be caused by hormones from COC, pregnancy and hormone replacement for menopause. After age 60, men have higher rates.

Men compared to similarly aged women who do not use COC, become pregnant or use hormone replacement still have an almost two-fold risk increase. Men are much more likely to have a second (or third) DVT compared to women. Men's rate is as high as 10 percent per year, while the rate for recurrence for women is 2 percent to 5 percent per year, indicated the study.

Women who have experienced a DVT may make different lifestyle choices, such as changing their type of contraception or not using hormone replacement therapy in menopause since they know they are at an increased risk of recurrence. This may partially explain the difference in recurrence. Men are also less likely to take the anticoagulant medication as prescribed after the first DVT.

As for erectile dysfunction—a concern for men when considering cardiovascular problems—Johnston said not to worry.

"Erectile dysfunction has more to do with the arterial blood flow, which is the other side of the circulatory pathway," he said. "There is no clear indication that a DVT can cause or worsen erectile dysfunction."