Serena Williams' Blood Clot After Childbirth: How Does It Happen?

that she experienced potentially life-threatening blood clots after giving birth to her daughter last year. But why does giving birth increase a woman's risk of blood clots?

Williams needed an emergency cesarean section to delivery her baby, Alexis Olympia Ohanian Jr., in September 2017, according to Vogue magazine. That surgery went well, but soon "everything went bad," Williams told Vogue.

Williams, who has a history of blood clots, started to have trouble breathing while recovering from her C-section. She thought she was having another blood clot, and told the medical staff she needed a CT scan and treatment for blood clots. Her doctors initially did not honor her request, and instead performed an ultrasound on her legs, Williams told Vogue. But eventually, they did give her a CT scan, which indeed showed that she had several blood clots in her lungs, known as pulmonary embolisms, according to Vogue. [10 Celebrities with Chronic Illnesses]

Williams was put on a life-saving blood-thinner medication, but this had the side effect of preventing her surgical C-section wound from healing properly. Her surgical wound reopened, and doctors performed another surgery in which they found a hematoma, or a mass of clotted blood, in her abdomen. She needed another operation to insert a filter into a major vein to prevent more clots, Vogue said.

It's known that a woman's risk of blood clots increases during pregnancy and shortly after giving birth. In fact, a woman's risk of blood clots is about four to five times higher when she is pregnant, compared with when she's not pregnant, said Dr. Saima Aftab, medical director of the Fetal Care Center at Nicklaus Children's Hospital in Miami, who is not involved with Williams' care.

There are several reasons for this. First, during pregnancy, women experience changes in levels of so-called blood-clotting factors, or proteins in the blood that help control bleeding, which makes pregnant women's blood clot more easily, Aftab said. What's more, as the uterus increases in size, it puts pressure on the blood vessels in the pelvis, which slows down blood flow to the legs, Aftab said. Additionally, women may not be able to get up and move around for long periods in late pregnancy, which slows down blood flow in the legs even more. All of these factors can increase the risk of blood clots, she said.

"Pregnancy is pretty much a very risky time period" for blood clots, Aftab told Live Science.

And this risk doesn't go away immediately after a woman gives birth — her risk for blood clots remains elevated for about six weeks after giving birth, according to the Centers for Disease Control and Prevention. That's because it takes time for the blood-clotting factors to return to normal, and for the uterus to reduce in size, Aftab said. And just as in late pregnancy, women may be less mobile in the first week or two after childbirth, she said.

Having a C-section further increases the risk of blood clots, as does any surgery. A 2016 study found that women who give birth via C-section are about four times more likely to develop blood clots, compared with women who give birth vaginally.

About 1 to 2 out of every 1,000 pregnant women experience a blood clot, according to Medscape. Women who have a history of blood clots, as Williams does, are at higher risk of blood clots during pregnancy and childbirth than women without a history of blood clots.

Blood clots in pregnancy tend to form in the deep veins of the legs or pelvis — a condition known as deep vein thrombosis, according to the American Society of Hematology. Pulmonary embolisms can occur when these clots break off and travel to the lungs.

The American College of Obstetricians and Gynecologists (ACOG) recommends that women with a history of blood clots use an anti-clotting medication to prevent blood clots during pregnancy. ACOG also recommends placing inflatable compression devices on a woman's legs during a C-section to prevent clots, and leaving them in place until the woman can walk again.

It's also important that pregnant women be on the lookout for signs of a blood clot, such as pain or swelling in the legs, particularly in the left leg, Aftab said.

And if patients experience shortness of breath and chest pain, doctors should take this seriously and evaluate them quickly, because it could be a sign of a life-threatening pulmonary embolism, Aftab said.