MDhealth.com explains that pink period blood is caused when normal discharge mixes with a small amount of blood. This type of bleeding typically occurs between periods and can be a sign of ovulation or the earliest stage of pregnancy.
According to MDhealth.com, if the pink blood is simply part of the normal menstrual cycle, it is followed by a flow that becomes heavier and darker gradually. This occurs when the menstrual blood is released lightly after ovulation fails to result in pregnancy. Pink discharge may also be a sign of pregnancy if a fertilized egg embeds itself into the uterine lining. In this case, MDhealth.com explains that the egg can cause some of the lining to break loose and leak out with the body's normal discharge, resulting in a pinkish appearance.
What to Expect explains that pink spotting may also be caused by the ovulation itself as the egg breaks free of the ovarian follicle, causing a small amount of blood. MDhealth.com also notes that in rare cases, pink blood can be a sign of infection or an STD and that women should seek medical attention if the other causes have been ruled out.