A substantial risk factor for future blood clotting events is a history of blood clots, particularly deep vein thrombosis (DVT) or pulmonary embolism (PE). Oestrogen, used in HRT, and oestrogen-containing drugs have been linked to an increased risk of blood clots. The dosage, oestrogen type (oral, transdermal, etc.), and other risk factors may all affect the risk in different ways but for the most part, HRT given transdermally through the skin, with micronized body identical progesterone, does not increase your risk of blood clots further and so this is generally what is recommended.
Those women with a history of clots or a family history of clots may need to have a pre-treatment blood test to look for any clotting disorder and the case may need to be discussed with a Haematologist but usually it does not prohibit use of HRT in the above formulation.