Say you had unprotected sex on the 16 and 18 of sept and got your period on the 21 when can would be a good time to take a pregnancy test to see if your pregnant?

Having a "period" or menstruating (medically speaking) means your body is eliminating the lining of the uterus, which is where the embryo will attach and form. If one does have a "period" AFTER the suspected impregnation, then it will generally not have been successful, meaning there is no pregnancy. However, to be certain, it is generally best to wait through two menstruation cycles (approximately two months) to be tested by a reliable party: Planned Parenthood and County Health Depts. are a couple sources of anonymous and reliable assistance in this.