NEW YORK (February 13, 2014) — Mothers remain at risk of developing a serious blood clot for 12 weeks after giving birth, rather than six weeks as previously thought, say a team of researchers in a study published online today in the New England Journal of Medicine.
While development of blood clots after childbirth is very uncommon, physicians should know that they can occur up to 12 weeks after delivery, and future studies are needed to see if some women will benefit from extended therapy to prevent such complications, says the study's lead investigator, Dr. Hooman Kamel, an assistant professor of neurology and neuroscience in Weill Cornell Medical College's Department of Neurology and Feil Family Brain and Mind Research Institute, and a neurologist at NewYork-Presbyterian Hospital.
Approximately 10 percent of postpartum stroke patients die of the disorder; Dr. Kamel estimates the same mortality rate occurs in new mothers who suffer heart attacks. Death from blood clots in the legs is uncommon, but the condition can cause pain and disfigurement, and can be potentially lethal if clots travel to the lungs, he says.
"When these events happen, there can be serious consequences, including long-term disability and death," says Dr. Kamel, who presented the findings at the International Stroke Conference of the American Heart Association in San Diego. "The risk of developing a clot is much higher than usual during the first six weeks after delivery, and we thought it was unlikely that the risk would suddenly return to normal at the end of six weeks, which is why we looked at what happens in the weeks following. It is important to know some women will continue to be at risk, and to search for ways that they can be effectively treated."
The study is unusual in several ways. It is the first to look, in a comprehensive manner, at thrombotic events from 0-12 weeks in new mothers, and it used a very large database to do so — researchers examined the medical records of almost 1.7 million women in California giving birth to their first child.
It found that 22 clot-related strokes, heart attacks or venous thromboembolisms (clots in the legs or lungs) occur within six weeks postpartum in every 100,000 deliveries, and another three clots occur in the next six weeks in 100,000 deliveries. After 12 weeks, the risk of clots is not significantly higher than the usual risk in women who are not pregnant.
"I often take care of patients who are in the neurological intensive care unit with stroke, and like my fellow co-authors, I have taken care of women who had these postpartum complications," says Dr. Kamel, who is a member of Weill Cornell's stroke and critical care division. "These are very distressing cases — first and foremost to the patients and families, but also to physicians taking care of them. We all remember these patients because the stakes are so high and the complications are so traumatic — far outside of the normal course of events."
Still, he adds, "It is important to stress that even if patients are at higher than usual risk for developing clots after giving birth, very few patients will actually have one of these complications. Almost every new mother is going to be fine."
Risk of clots falls off after 12 weeks
Obstetricians monitor their patients throughout pregnancy for risk of blood clots. Women at the highest risk are those who have chronic high blood pressure, those who develop high blood pressure during pregnancy, older women, smokers, women who have had a blood clot before, and women with certain inherited blood disorders.
Clots can develop in pregnant women because their blood is more likely to clot in order to prevent excessive blood loss during labor and childbirth. Additionally, pregnancy hormones cause blood to flow more slowly than usual from the veins in a woman's legs to her heart.
To study how many new mothers experience a clot in the 7-12 weeks after birth, the researchers developed a method to assess the risk of such events in each of 1,687,930 women during the 12 weeks after delivery and compare it to the same period one year following delivery.
"This method isolated thrombotic risk that was due to pregnancy itself and not other health conditions," Dr. Kamel says.
Current guidelines call for use of low-dose, preventive blood thinners in especially at-risk women for six weeks after delivering. Dr. Kamel says additional research should compare the outcome of postpartum use of blood thinners in these high-risk patients for six versus 12 weeks to determine whether the treatment should last longer.
Working with Dr. Kamel on the study were, from Weill Cornell Medical College, Dr. Babak B. Navi, Nandita Sriram, Dominic Hovsepian and Dr. Richard Devereux; and Dr. Mitchell S. V. Elkind from Columbia College of Physicians and Surgeons.
The study was funded by a grant, K23NS082367, from the National Institute of Neurological Disorders and Stroke.
Weill Cornell Medical College
Weill Cornell Medical College, Cornell University's medical school located in New York City, is committed to excellence in research, teaching, patient care and the advancement of the art and science of medicine, locally, nationally and globally. Physicians and scientists of Weill Cornell Medical College are engaged in cutting-edge research from bench to bedside, aimed at unlocking mysteries of the human body in health and sickness and toward developing new treatments and prevention strategies. In its commitment to global health and education, Weill Cornell has a strong presence in places such as Qatar, Tanzania, Haiti, Brazil, Austria and Turkey. Through the historic Weill Cornell Medical College in Qatar, Cornell University is the first in the U.S. to offer a M.D. degree overseas. Weill Cornell is the birthplace of many medical advances — including the development of the Pap test for cervical cancer, the synthesis of penicillin, the first successful embryo-biopsy pregnancy and birth in the U.S., the first clinical trial of gene therapy for Parkinson's disease, and most recently, the world's first successful use of deep brain stimulation to treat a minimally conscious brain-injured patient. Weill Cornell Medical College is affiliated with NewYork-Presbyterian Hospital, where its faculty provides comprehensive patient care at NewYork-Presbyterian Hospital/Weill Cornell Medical Center. The Medical College is also affiliated with Houston Methodist. For more information, visit weill.cornell.edu.