Bleeding between periods, also known as breakthrough bleeding, has many causes, according to WebMD. Mid-cycle bleeding often is associated with normal ovulation, and many women experience small amounts of bleeding between periods when they are taking birth control pills. Other causes include polycystic ovary syndrome, uterine fibroids, intrauterine devices and infections, including sexually transmitted diseases and pelvic inflammatory disease.
According to Baby Corner, ovulation bleeding is a common and normal occurrence. During a normal menstrual cycle, the release of an egg into the fallopian tube, or ovulation, occurs anywhere from day 11 to day 21. At this time, hormones cause the ovarian follicle to weaken and allow an egg to break through. Sometimes, the rupture of the follicle causes minor bleeding, which appears as pinkish or brown spotting. Ovulation bleeding is sometimes accompanied by pain over the right or left ovary, a condition known as mittelschmerz, which is German for middle pain.
Taking birth control pills is another frequent cause of breakthrough bleeding, according to the Mayo Clinic. Intermittent bleeding is more common with pills that are taken on a continuous or extended cycle, such as levonorgestrel and ethinyl estradiol tablets (Seasonale, Seasonique), than with pills taken for 28 days. Smoking cigarettes, missing a dose of your medicine or taking antibiotics also may contribute to intermittent bleeding. Generally, the bleeding decreases over time if you continue taking the pills.
With few exceptions, the Mayo Clinic advises women to see their doctors for any abnormal vaginal bleeding. This is important to rule out serious medical conditions, such as infections, uterine fibroids, hormonal irregularities and tumors, and to monitor the bleeding over time.