**Core Concept**
The patient's symptoms of breathlessness, history of recent surgery, and pregnancy in the 3rd trimester suggest a condition related to **venous thromboembolism**. This is a concern due to the hypercoagulable state associated with pregnancy and the increased risk of thrombosis following surgery.

**Why the Correct Answer is Right**
Given the patient's recent surgery and pregnancy, she is at an increased risk for **pulmonary embolism** due to **deep vein thrombosis (DVT)**. The hypercoagulability of pregnancy, combined with the immobilization from surgery, elevates the risk of clot formation. This clot can break loose, traveling to the lungs and causing a **pulmonary embolism**.

**Why Each Wrong Option is Incorrect**
**Option A:** Amniotic fluid embolism is a rare but serious condition that occurs when amniotic fluid enters the maternal circulation, typically during childbirth or after a traumatic event. It is less likely given the context of recent surgery and the patient's current symptoms.
**Option C:** Air embolism could occur in a medical setting, especially with invasive procedures, but the patient's presentation and history do not strongly suggest this as the primary concern.
**Option D:** Fat embolism is often associated with long bone fractures, but the patient's symptoms and the context provided do not primarily point towards this diagnosis.

**Clinical Pearl / High-Yield Fact**
Pregnancy increases the risk of venous thromboembolism due to the hypercoagulable state, and any recent surgery further elevates this risk. Thus, pregnant patients who have undergone surgery should be closely monitored for signs of DVT or pulmonary embolism.

**Correct Answer:** D. Pulmonary embolism