Bleeding between periods, sometimes called intermenstrual bleeding or spotting, is common and usually has a treatable cause. Hormonal shifts, birth control side effects, infections, and growths like polyps or fibroids top the list. While most causes are benign, unexpected bleeding is worth paying attention to because it can occasionally signal something more serious.
Hormonal Birth Control Is the Most Common Culprit
If you recently started hormonal contraception, spotting is almost expected. About 30 percent of women experience abnormal bleeding during the first month on combination birth control pills, and the rate drops significantly by month three. This “breakthrough bleeding” happens because your uterine lining is adjusting to the new hormone levels keeping it thin.
Spotting is also common if you skip pills, take them at inconsistent times, switch brands, or stop and restart your birth control. Hormonal IUDs frequently cause irregular spotting in the first three to six months after insertion. If you’re past that adjustment window and still bleeding, the type or dose of your contraception may need to change.
Hormone Fluctuations Outside of Birth Control
Your menstrual cycle depends on estrogen and progesterone rising and falling in a predictable pattern. Anything that disrupts that rhythm can cause your uterine lining to shed at the wrong time, resulting in spotting. An underactive thyroid is one well-known disruptor: low thyroid hormone output can throw off your cycle length and trigger bleeding between periods.
Chronic stress works through a different pathway but produces a similar result. Sustained high cortisol levels interfere with the hormone signals your brain sends to your ovaries, disrupting the development of egg follicles and sometimes preventing ovulation altogether. When ovulation doesn’t happen on schedule, progesterone levels stay low, and the uterine lining can break down unevenly, causing spotting mid-cycle.
Perimenopause is another major source of hormonal unpredictability. As the ovaries gradually produce less estrogen, cycles become irregular. Periods may come closer together, further apart, or with spotting in between. This transition typically begins in your 40s but can start earlier. Bleeding patterns during perimenopause vary widely from person to person, so there’s no single “normal” to compare yourself against.
Polyps and Fibroids
Uterine polyps are soft growths that develop in the inner lining of the uterus. They attach to the endometrium by a thin stalk or a broad base and range from the size of a sesame seed to as large as a golf ball. Intermenstrual bleeding and spotting after intercourse are hallmark symptoms. Polyps are usually noncancerous, but a small percentage (around 3 percent in one large study) show precancerous or cancerous changes, particularly in postmenopausal women with abnormal bleeding.
Fibroids are muscular growths in the uterine wall. They’re extremely common, especially during reproductive years, and their effect on bleeding depends largely on their location. Fibroids that grow into the uterine cavity or distort its lining are the ones most likely to cause spotting, heavy periods, or prolonged bleeding. Fibroids that sit on the outer surface of the uterus often cause no bleeding symptoms at all.
Infections and Inflammation
Sexually transmitted infections, particularly chlamydia and gonorrhea, can cause bleeding between periods by inflaming the cervix. The cervix becomes fragile and irritated, which means it bleeds easily, sometimes on its own and sometimes after sex. You may also notice an unusual discharge or a dull ache in your lower abdomen, though many people with these infections have no symptoms beyond the spotting itself.
If left untreated, these infections can spread upward from the cervix into the uterus and fallopian tubes, causing pelvic inflammatory disease (PID). PID symptoms often appear toward the end of a menstrual period or in the days just after, and include irregular vaginal bleeding, lower abdominal pain (sometimes worse on one side), and a discharge that may have a bad odor. PID is treatable with antibiotics but can cause lasting damage to reproductive organs if it’s not caught early.
Pregnancy-Related Causes
Unexpected bleeding can sometimes be the first sign of pregnancy. Implantation bleeding, a small amount of spotting that occurs when a fertilized egg attaches to the uterine lining, happens roughly 10 to 14 days after conception and is easy to mistake for an early or light period. It’s typically light pink or brown and lasts a day or two.
More concerning pregnancy-related causes include miscarriage and ectopic pregnancy, where the embryo implants outside the uterus (usually in a fallopian tube). Ectopic pregnancy is a medical emergency. If there’s any chance you could be pregnant and you’re experiencing bleeding with sharp or worsening pelvic pain, or if you feel dizzy or faint, seek care immediately.
Less Common but Serious Causes
Cancers and precancers of the cervix or uterus can cause intermenstrual bleeding. This is not the most likely explanation for spotting, but it’s the reason doctors take persistent or unexplained bleeding seriously, especially in certain risk groups. Postmenopausal women with abnormal bleeding carry a significantly higher risk: one study found they had a 20-fold greater chance of precancerous or cancerous findings compared to other groups. Being up to date on cervical cancer screening (Pap tests and HPV testing) is one of the most straightforward ways to catch cervical changes early.
Blood-thinning medications can also cause intermenstrual bleeding by making it harder for your body to stop small amounts of normal uterine shedding.
How Doctors Investigate the Cause
The evaluation typically starts with your medical history, a physical exam, and sometimes blood work to check hormone levels, thyroid function, or pregnancy status. If a structural cause like polyps or fibroids is suspected, transvaginal ultrasound is usually the first imaging step. It’s a quick, non-invasive scan that gives a good view of the uterine lining and any growths.
If the ultrasound isn’t definitive, or if a growth needs a closer look, your doctor may recommend hysteroscopy, a procedure where a thin camera is inserted through the cervix to examine the uterine cavity directly. When ultrasound and hysteroscopy are used together, they correctly identify endometrial polyps about 80 percent of the time. For women at increased risk of endometrial cancer, based on age, weight, or a history of irregular cycles, an endometrial biopsy may be done to check for abnormal cell changes.
Signs That Need Prompt Attention
Most intermenstrual spotting turns out to be benign, but certain patterns call for urgent evaluation. Soaking through a pad or tampon every hour for two to three hours signals heavy blood loss. Bleeding paired with fever or severe lower abdominal pain could point to infection or an ectopic pregnancy. Feeling weak or lightheaded alongside bleeding suggests you’re losing enough blood to affect your circulation.
Any bleeding or spotting after menopause warrants a visit to your provider, as does bleeding that lasts longer than a week, bleeding that keeps returning, or spotting that consistently happens after sex. These don’t necessarily mean something dangerous is happening, but they’re the situations where a clear diagnosis matters most.