You are at a greater risk of having a miscarriage if you:-

http://www.patient.co.uk/health/miscarriage-and-bleeding-in-early-pregnancy

 Smoke. Smoking more than 14 cigarettes per day doubles the risk compared with non-smokers.
Drink too much alcohol. Even drinking four units of alcohol a week (one unit is half a pint of beer or a small glass of wine) has been shown to increase the risk of miscarriage.
Use recreational drugs.