If you have had regular 28-30 day menstrual cycles and do not have a period as due, you should be knowing that. Unless you continue to menstruate throughout ( as happens in some cases especially when there is a bicornuate uterus
) it is impossible to know when you are pregnant. You should be knowing your last menstrual period and you can get checked by a gynecologist and have a sonogram to evaluate the situation. Take care.