If you take contraceptive correctly and there was no failure in the method, such as tablet oblivion or delay to take the injection, the possibility of pregnancy is very low, less than 1% of chance.
Women who use contraception have reduced monthly bleeding or even no menstruation. This happens because the contraceptive hormone tries to mimic the woman's menstrual cycle but in a way that does not occur ovulation and consequently pregnancy. In this way, the inner layer of the uterus (endometrium) is constantly thin and often there is nothing to flay and in these cases no menstruation occurs.
If the contraceptive use is not correct or has failed to use, there is a possibility of pregnancy.
For more information: Contraceptive questions