According to NHS Choices:

Each year, DVT affects around one person in every 1,000 in the UK.
Anyone can develop DVT, but it becomes more common over the age of 40. As well as age, there are also a number of other risk factors, including:

• having a history of DVT or pulmonary embolism
• having a family history of blood clots
• being inactive for long periods – such as after an operation or during a long journey
• blood vessel damage – a damaged blood vessel wall can result in the formation of a blood clot
• having certain conditions or treatments that cause your blood to clot more easily than normal – such as cancer (including chemotherapy and radiotherapy treatment), heart and lung disease, thrombophilia and Hughes syndrome
• being pregnant – your blood also clots more easily during pregnancy
• being overweight or obese

The combined contraceptive pill and hormone replacement therapy (HRT) both contain the female hormone oestrogen, which causes the blood to clot more easily. If you're taking either of these, your risk of developing DVT is slightly increased.

What are the limitations of DVT Ultrasound scan Imaging?

It can be hard to fully visualise the internal anatomy of large patients as the sound has further to travel and therefore the returning echoes are weaker

Veins lying deep beneath the skin, especially small veins in the calf, may be hard to see.

Venous ultrasonography is accurate for detecting blood clots in the veins of the upper leg (duplex ultrasound identifies 95% of the clots) but is less accurate in the calf veins, as the veins become very small. DVT ultrasound identifies up to 70% of calf vein DVT. Potentially dangerous venous clots are however typically found in the larger femoral veins and calf vein thrombi are less likely to break and travel to the lungs to cause pulmonary embolism