**Core Concept**
Pregnancy increases the risk of venous thromboembolism (VTE) due to increased blood volume, venous stasis, and hypercoagulability. In the third trimester, the risk of deep vein thrombosis (DVT) and pulmonary embolism (PE) is particularly high.

**Why the Correct Answer is Right**
During pregnancy, the increased levels of estrogen cause a hypercoagulable state, which predisposes to the formation of blood clots. In the third trimester, the pressure on the veins in the legs from the expanding uterus can lead to venous stasis, further increasing the risk of DVT. The most common type of thrombosis in pregnancy is a DVT, typically occurring in the proximal veins of the leg, such as the popliteal or femoral veins.

**Why Each Wrong Option is Incorrect**
* **Option A:** Arterial thrombosis is less common in pregnancy and typically occurs in the context of pre-existing vascular disease or hypertension.
* **Option B:** Pulmonary embolism is a complication of DVT, but it is not the type of thrombosis itself.
* **Option D:** Thrombophlebitis refers to inflammation of a vein accompanied by clot formation, but it is not a specific type of thrombosis.

**Clinical Pearl / High-Yield Fact**
In pregnant patients, the risk of VTE is increased by factors such as immobilization, obesity, and a history of previous DVT or PE. A high index of suspicion is essential for early diagnosis and treatment to prevent maternal and fetal morbidity and mortality.

**Correct Answer:** C. Deep vein thrombosis (DVT).