Spotting between periods is not uncommon and can have many different causes. In most cases, spotting represents the flow of only a small amount of blood--hence the term "spotting"--which typically takes a while to pass from the uterus to the exterior of the vagina. Fresh blood has a bright red color, but spotting is often brownish in hue because it's less fresh and has oxidized. True spotting consists of blood, however, and should therefore have very little odor.
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One of the possible reasons for spotting between expected periods is pregnancy, explain Heidi Murkoff and Sharon Mazel in their book, "What To Expect When You're Expecting." When a woman conceives, it takes seven to 10 days for the fertilized egg to travel from the site of conception in the Fallopian tube to her uterus. Once it reaches the uterus, the egg burrows into the uterine lining, which causes a small amount of bleeding. In some women, this bleeding never reaches the outside of the vagina, but others do notice some spotting about a week before their expected period. If the subsequent expected menstrual cycle fails to arrive, it may be worth taking a pregnancy test.
Birth Control Pills
Oral birth control pills contain small amounts of synthetic hormones that mimic the action of estrogen and progesterone. Women typically take active, hormone-containing pills for 21 days, then take placebo pills for the next seven days. The falling hormone levels during placebo days trigger a menstrual period. The McKinley Health Center explains, however, that falling hormone levels can trigger periods, or spotting, at other times as well. If a woman fails to take her pill at the same time each day, for instance, the body can interpret the late pill as falling hormone levels, which can initiate bleeding. Once levels start to rise again, the bleeding stops, producing symptoms of spotting. Women can avoid spotting between periods by being very consistent with their pills.
Stress and Disease
MedlinePlus, maintained by the U.S. National Library of Medicine, notes that spotting between periods may also indicate physiological or psychological problems. If a woman is severely stressed, her brain may alter its hormone production. This can cause periods to become more or less frequent, increase or decrease in volume, and may further cause spotting between periods. In general, as the stress resolves, spotting and other symptoms subside. Very rarely, spotting can indicate uterine fibroids, cancer, or other disease processes. Generally, however, these cause other symptoms as well--such as altered menstrual period frequency and duration--and pain. If spotting is minor, isn't accompanied by pain and subsides within a few days, it's highly unlikely that it's the result of a significant health disorder.