ORLANDO, FLA. — Having a child cuts a woman's risk of unprovoked venous thromboembolism, according to results of an epidemiologic study with more than 19,000 women.
Compared with women who were never pregnant, women with a history of at least one pregnancy had a 41% reduced risk of venous thromboembolism (VTE) in a multivariate model that was adjusted for several potential confounders, Christiana Iyasere, M.D., told attendees during the annual meeting of the American College of Cardiology.
The difference in risk was statistically significant, said Dr. Iyasere, a cardiologist at Brigham and Women's Hospital in Boston.
The overall incidence of unprovoked VTE in nulliparous women was about 0.8%, compared with a rate of about 0.5% in women who had at least one pregnancy.
This means that, on average, a history of pregnancy cut the population risk for unprovoked VTE by about 3 cases per 1,000 people.
The study used data collected in the Women's Health Study, which recruited postmenopausal women in the health professions with a primary goal of assessing the safety and efficacy of both aspirin and vitamin E for the primary prevention of cardiovascular disease.
Dr. Iyasere and her associates used data collected by questionnaire from the nearly 40,000 women who participated in this study to evaluate the impact of endogenous hormones on VTE risk.
They excluded women with a history of VTE, known hypercoagulable states, a history of prolonged immobility, a history of malignancy, or recent trauma.
The analyses were done using data collected from the remaining 19,219 women.
The researchers found no detectable association between VTE risk and age of menarche, age of menopause, and total duration of menses.
There was also no significant association between VTE risk and parity when the women were subdivided into four categories based on their child-bearing history: nulliparous, one or two pregnancies, three or four pregnancies, or five or more pregnancies. But a statistically significant association existed when the comparison was made between nulliparous women and those with any pregnancy.
Among 2,635 nulliparous women, 21 reported having an unprovoked VTE.
Among 16,584 women who were pregnant at least once, 86 had an unprovoked VTE.
The fully adjusted multivariate model corrected the hazard ratio for age, history of exogenous hormone use, body mass index, history of diabetes, history of hypertension, history of smoking, and degree of exercise activity.
The researchers were surprised to find no apparent link between other measures of endogenous hormone exposure and VTE risk because exogenous hormones, in the form of oral contraceptives and hormone therapy, are proven risk factors for an increased VTE incidence, Dr. Iyasere commented at the meeting.