Ante- and postnatal risk factors of venous thrombosis: a hospital-based case-control study

Objective: To study ante- and postnatal risk factors of venous thrombosis (VT) in pregnancy. Methods: A hospital-based case-control study. Cases were women with objectively verified VT during pregnancy or postpartum. Two controls were selected for each case. Validated risk factors were analyzed using chi-square test and logistic regression. Results: In total 559 cases with no prior VT, 268 ante- and 291 postnatal cases were identified together with 1229 controls. Risk factors for antenatal VT were assisted reproduction technique (ART), antepartum immobilization, cigarette smoking, and slight weight gain (< 7 kg). Conception after ART and multiple pregnancy had an additive effect, whereas antepartum immobilization and high body mass index (BMI) had a multiplicative effect on the risk for antepartum VT. No other interaction was found between risk factors for antepartum VT. Risk factors for postnatal VT were antepartum immobilization, cigarette smoking, intrauterine fetal growth restriction (IUGR), preeclampsia, emergency cesarean section, postpartum hemorrhage, infection, surgery, and age and parity. Antepartum immobilization, high BMI and reoperation on the indication of bleeding showed multiplicative effects on the risk of postnatal VT. Conclusions: Ante- and postpartum risk factors differed markedly. More attention should be paid to pregnant women of high BMI who are immobilized.