The Vein Treatment Center is a National Center of Excellence for Venous Disorders and DiseasesTM
Physiologic Changes of Clotting Factors During Pregnancy - Risk factors for DVT in pregnancy
Physiologic changes of clotting factors during pregnancy include:
Elevated circulating levels of Factor VII, Factor VIII, Factor X and Factor XII during pegnancy increase risk of developing venous thromboembolism. This risk is highest right after delivery of the baby (the puerperium).
There are also a reduction in sensitivity to activated protein C and functional protein S levels during pregnancy.
The risk of develoing venous thromboembolism during pregnancy is 0.66 per 100.
This data is from P Kesteven and colleagues and the article is published in Phlebology 2012; volume 27 Supplement: pages 73-80.

Endoscopic view of blood clot across a vein valve (venous endoscopy):

courtesy of Prof Alberto Caggiati (Rome, Italy)
Inherited conditions can lead to a genetic predisposition to develop venous thrombotic events (deep vein clots):
Factor V Leiden mutation (most common)
Activated protein C mutation
Antithrombin mutations
Protein C mutations
Protein S mutations
Elevated Factor VIII levels
Prothrombin mutations
DysFibrinogenemia
These conditions are frequently the cause of clot formation in young individuals (< 45 years) and may contribute to deep vein clots at any age. To learn more about these conditions, or if you have developed a deep vein clot at an early age - or have had recurrent deep vein clots (more than 1), consult your doctor to get evaluated for these inherited conditions using a simple blood test.
Chemotherapy and Radiation Therapy increase the risk of Deep Vein Thrombosis
Venous Thromboembolism and Cancer Type
Treatment of DVT and Venous Thromboembolism in Patients with Cancer
Anticoagulation for DVT thrombophylaxis after hip surgery and knee surgery
Antiphospholipid Syndrome and Venous Thromboembolism
Hemorrhagic Complications with Anticoagulation for DVT and Venous Thromboembolism
Hormone replacement therapy and venous thromboembolism
Xarelto for Deep Vein Thrombosis - rivaroxaban for DVT
Protein C Deficiency and Deep Vein Thrombosis, mutations PROC gene
Thrombophilia and Exercise, Smoking Cessation, Avoiding Immobility, Surgery, Oral Contraceptives, Pregnancy
Thrombophilia and Relative Risk for DVT and Venous Thromboembolism
Ways to Reduce Risk of Deep Vein Thrombosis (DVT)
What is Travelers DVT (Deep Vein Thrombosis)?
Deep Vein Thrombosis is Multifactorial
Three Ways to Diagnose Deep Vein Thrombosis
Are Temporary Vena Cava Filters Really Temporary?
Blood Clots During Pregnancy and Risk factors for Deep Vein Thrombosis During pregnancy