Risk factors for venous thromboembolism during pregnancy.

Background: Pregnant women are at an increased risk of venous thromboembolism (VTE). Risk factors for VTE among pregnant women are not sufficiently investigated.

Purpose: To examine pharmacological and non-pharmacological VTE risk factors during pregnancy (antepartum).

Methods: The population comprised all pregnant women in Denmark aged 15-50 giving birth 2003-2010. Pregnancies were linked on an individual level with national registers for hospital admissions and drug dispenses from pharmacies. Risk of first occurring VTE antepartum was examined with Cox regression models.

Results: Out of 299 810 pregnancies, 337 experienced a VTE, incidence rate 1.1 (95% confidence interval [CI] 1.0-1.3) per 1000 pregnancies. Being underweight (body mass index [BMI] < 18.5 kg/m(2) ) was associated with a decreased risk of VTE (hazard ratio [HR] 0.53 [CI 0.29-0.98]) compared to normal weight (18.5 ≤ BMI < 25 kg/m(2) ). Overweight (25 ≤ BMI < 30 kg/m(2) ) increased VTE risk (HR 1.30 [CI 1.01-1.67]) but obesity (BMI ≥ 30 kg/m(2) ) was insignificant (HR 1.14 [CI 0.82-1.58]). A history of VTE was highly significant (HR 72.65 [CI 51.17-103.15]). The youngest (<20 years) and oldest (≥35 years) had insignificantly increased risks (HR 1.45 [CI 0.80-2.62] and HR 1.31 [CI 0.98-1.75], respectively) compared to those aged 20-30 years. Sixteen groups of medications, including anti-infectious medications, hormones, aminosalicylic acid, insulin, and benzodiazepine derivatives, were associated with VTE.

Conclusion: The risk of antepartum VTE was increased in women with prior VTE. Compared to normal weight women, being underweight decreased the risk of VTE whereas being overweight increased the risk. Also, the use of several medications was associated with increased risk of VTE.