Most women become accustomed to a regular routine that indicates their menstrual cycle is progressing normally, with menstrual flow happening once each month. Women taking birth control pills or using devices containing birth control medication might or might not have monthly periods. In either case, breakthrough bleeding may occasionally occur. This is uterine bleeding occurring at any time other than when a woman expects her period. Although breakthrough bleeding has several possible causes, it's usually not a cause for serious concern. In some instances, however, it can indicate a problem that should be discussed with your doctor.
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During a Normal Cycle
During a normal menstrual cycle, changes in hormone production by the ovary triggers shedding of part of the uterine lining and menstrual flow. In adolescent girls who've just begun to menstruate, breakthrough bleeding or light midcycle spotting is common. This usually resolves after the first several cycles. Some women of childbearing age experience excessive menstrual bleeding even though their cycles are normal. In these women, breakthrough bleeding might also occur at other times during their cycles. This has several possible causes, including the presence of benign growths in the uterus, called fibroids or polyps, which can be treated if breakthrough bleeding or heavy periods become a problem.
Breakthrough bleeding might occur in women who don't ovulate in each cycle. During one of these anovulatory cycles, low levels of sex hormones during the first half of the cycle fail to bring an egg to full development. The low hormone levels also disrupt the normal cyclic formation of the uterine lining. This can lead to bleeding at one or more points before the normal time of menstrual flow. Older women who are nearing the age of menopause and are in a transitional period called perimenopause might also experience breakthrough bleeding. During perimenopausal cycles, unusual bleeding or spotting can occur at any point of the cycle, generally indicating that hormone levels are below normal and ovulation may not have occurred. Eventually, perimenopause is followed by complete cessation of menstruation, when menopause begins.
Birth Control Pills
Breakthrough bleeding sometimes occurs in women taking birth control pills or using other types of birth control methods involving medication, especially during the first few months of treatment. Birth control pills and medicated devices contain female hormones in various formulations. Women using these contraceptives may experience regular monthly periods, have flow only once every several months or not have any flow, depending on the type of birth control treatment. Sometimes, the uterine lining might bleed at unusual times during one or more cycles in these women, causing breakthrough bleeding. Women who smoke are more likely to experience breakthrough bleeding while on hormonal birth control, as are those who miss one or more doses of medication in a given cycle. In many cases where missing a dose isn't the cause, these instances of breakthrough bleeding become less frequent or disappear as the woman's body adjusts to the birth control medication.
In rare instances, breakthrough bleeding or occasional spotting might signal the existence of an underlying condition that causes abnormal production of female hormones. These include hypothyroidism, which involves low production of thyroid hormone; high levels of another hormone called prolactin; or abnormal buildup of the uterine lining, a condition called endometriosis. A uterine infection or cancer of the cervix or uterine lining could also cause abnormal bleeding or spotting.
When to Contact Your Doctor
If you experience frequent breakthrough bleeding or bleeding at an abnormal time that becomes heavy or doesn't stop after a few days, or if you have questions about breakthrough bleeding, talk with your doctor.