If i have sex in march and get pregnant will i still have a period for march if it hasnt came yet ?

Possible: Some women will experience a period in the first few months of pregnancy.

No: If already pregnant, vaginal bleeding would not be a woman's regular period. It means the perceived "period" during pregnancy is not a real menstrual period, because having a real menstrual period clears out the uterus... So having a real menstrual period means not pregnant at that time.

Some bleeding poss: Up to one third of women may have some traces of vaginal bleeding, and most go on to have a full-term pregnancy. But I agree that a "normal" period is unlikely.