You can’t usually get pregnant if you’re not 12 years old and haven’t had your period.
The first condition for pregnancy is the combination of sperm and egg to form a fertilized egg and to implant it.
The fact that you have not menstruated before the age of twelve means that you have not yet developed, the ovaries are not yet fully developed, and there is no ovulation, so if you have sex, the sperm will not be able to combine with the egg, and you will not get pregnant.
The girl generally in 12-15 years old began to develop, if already have menstruation, ovarian function began to ovulate, as long as there is sexual intercourse may lead to accidental pregnancy.
Not yet 12 years old belongs to the underage, too young, the body and reproductive organs have not yet developed, premature sex will affect the development of the reproductive system, jeopardizing health, to strictly prohibit sex.