I'm A 34 Year Old Female With Factor V Leiden

I'm a 34 year old female with factor v Leiden (Heterozygous). I am overweight and about to travel to USA (flight 1=4 hours, 2 hrs transit; flight 2=16 hrs, 4 hours transit; flight 3= 2 hrs). My dr prescribed clexane injection the day of the flight and compression stockings thigh high. But my sister dr said she has to take the same injection the day before flight, day of and day after flight. We have exactly the same condition and both no history or dvt or clots.
How many injection and when should we take them? Prescribed dosage is 20mg per injection. Thanks