Where does the blood come from when I have my period?

Every month the womb prepares for the implantation of a fertilized egg: the spongy tissue fills with blood. In between two periods an egg cell is released (ovulation). But when the egg cell doesn’t meet a sperm cell it dies within 48 hours, so the girl won’t become pregnant. The preparation of the womb was vain: the lining is shed. This leaves the body through the vagina, coming out as a slimy, bloody fluid: you have your period. After this the whole cycle starts again. So the blood you see is the lining of the womb. If you take a good look, for instance when you take a shower and see the blood against the white of the shower tray, you can see it is not just blood, but bloody tissue. If fertilization does take place the fertilized egg is implanted in the womb and the lining is not shed. So missing a period is normally the first sign of pregnancy.