Bleeding between periods has a wide range of causes, from completely harmless mid-cycle spotting to conditions that need treatment. Most of the time, the explanation is something benign like hormonal shifts, a new contraceptive, or ovulation. But because intermenstrual bleeding can also signal infections, growths, or pregnancy complications, it’s worth understanding the possible reasons so you know what to pay attention to.
Ovulation Spotting
The most common harmless cause of mid-cycle bleeding is ovulation. In a typical 28-day cycle, ovulation happens around day 14. Right after the egg is released, estrogen levels drop briefly, and for some people that small hormonal dip causes a thin layer of the uterine lining to shed. The result is light pink or brownish spotting that lasts a day or two at most.
This kind of spotting is faint enough that you’d notice it on toilet paper or as a small spot on underwear, not enough to fill a pad. If your mid-cycle spotting follows a predictable pattern each month and stays light, ovulation is the likely explanation.
Hormonal Contraceptives
Starting or switching birth control is one of the most frequent triggers for unexpected bleeding. Up to 30% of people experience irregular bleeding in the first month on combination birth control pills. With contraceptive implants, that number climbs to 80% during the first year. Hormonal IUDs also commonly cause spotting in the early months of use.
This happens because your body is adjusting to new hormone levels. The bleeding is typically light and settles down within three to six months. Missing a pill or taking it at inconsistent times can also trigger spotting because the brief hormone fluctuation destabilizes the uterine lining. If breakthrough bleeding persists beyond the first few months, your prescriber may suggest a different formulation or dose.
Infections and STIs
Sexually transmitted infections, particularly chlamydia and gonorrhea, can cause bleeding between periods. These infections inflame the cervix, making its surface fragile and prone to bleeding, especially after sex. Chlamydia specifically lists vaginal bleeding between periods and after intercourse among its symptoms.
Left untreated, these infections can progress to pelvic inflammatory disease (PID), an infection that spreads to the uterus and fallopian tubes. PID can damage reproductive organs and often brings additional symptoms like pelvic pain, unusual discharge, and fever. Because chlamydia frequently causes no obvious symptoms early on, unexplained spotting in someone who is sexually active is a good reason to get tested.
Uterine Polyps and Fibroids
Polyps are small, finger-like growths on the uterine lining. They’re surprisingly common: roughly 10 to 15% of people with uteruses have them even without any bleeding symptoms. Among those who do have abnormal bleeding, the prevalence rises to 20 to 30%. Polyps are recognized by the International Federation of Gynecology and Obstetrics as a specific cause category for abnormal uterine bleeding, and removing them often resolves the problem.
Fibroids, which are noncancerous growths in the muscular wall of the uterus, can also cause bleeding between periods. Fibroids that grow into the uterine cavity (rather than outward) are the most likely type to disrupt bleeding patterns. Both polyps and fibroids become more common with age, which is one reason bleeding irregularities increase as people approach their 40s.
Pregnancy-Related Bleeding
If there’s any chance you could be pregnant, bleeding between expected periods takes on a different significance. A fertilized egg implanting in the uterine lining can cause light spotting roughly 10 to 14 days after conception. This implantation bleeding is typically brown or light pink and very brief.
However, bleeding during early pregnancy can also indicate a problem. About one-third of people bleed during the first trimester, and roughly half of those pregnancies will end in miscarriage. Warning signs of miscarriage include bright red bleeding or clots, cramping, passage of tissue, and pregnancy symptoms like nausea and breast tenderness fading suddenly. Brown discharge that looks like coffee grounds can be old blood leaving the uterus slowly and isn’t always an emergency, but it still warrants evaluation.
Ectopic pregnancy, where the embryo implants outside the uterus (usually in a fallopian tube), can also present with vaginal bleeding and is potentially life-threatening. If you have bleeding alongside sharp or one-sided pelvic pain and dizziness, seek immediate care.
Perimenopause
For people in their 40s and early 50s, irregular bleeding is often one of the first signs of perimenopause. As ovarian function becomes less predictable, hormone levels fluctuate more dramatically from month to month. Some cycles you may ovulate, others you won’t, and the lining of the uterus builds and sheds on an erratic schedule. This is considered part of normal physiology during the menopausal transition.
That said, perimenopausal bleeding deserves extra attention because the risk of polyps, fibroids, and uterine abnormalities also increases with age. Once you’ve fully reached menopause (12 consecutive months without a period), any bleeding is considered abnormal. Roughly 9% of postmenopausal people who see a doctor for bleeding are eventually diagnosed with endometrial cancer. The rate varies geographically, from about 5% in North America to 13% in Western Europe, but it’s high enough that postmenopausal bleeding always warrants prompt evaluation.
Less Common Causes
Thyroid disorders can disrupt the hormonal chain that regulates your cycle, leading to spotting or irregular periods. Both an overactive and underactive thyroid can cause this. Bleeding disorders like von Willebrand disease, which affects the blood’s ability to clot, are another possibility, particularly in people who have had heavy or irregular bleeding since their first periods. Polycystic ovary syndrome (PCOS), which involves chronic anovulation and prolonged estrogen exposure, is also a well-established cause of unpredictable bleeding.
What Evaluation Looks Like
If you bring up bleeding between periods, expect a pelvic exam first. This involves a speculum and manual examination to check for visible causes like cervical inflammation, trauma, or an enlarged uterus that might suggest fibroids. A pregnancy test is standard for anyone of reproductive age.
Depending on your history, additional steps may include blood work to check thyroid function or clotting ability, STI testing, and a pelvic ultrasound to look for structural issues like polyps or fibroids. For people over 45, an endometrial biopsy (a tissue sample from the uterine lining) is typically the first-line test to rule out precancerous or cancerous changes. People under 45 may also need a biopsy if they have risk factors like obesity, PCOS, or bleeding that hasn’t responded to initial treatment.
When Bleeding Is an Emergency
Most intermenstrual bleeding is light and not dangerous. But if you’re soaking through a pad or tampon every hour for more than two hours in a row, and you also feel dizzy, lightheaded, short of breath, or have chest pain, that combination requires emergency care. Heavy blood loss can cause dangerous drops in blood pressure, and these symptoms suggest your body is struggling to compensate. Bleeding with severe pelvic pain and a positive pregnancy test is also an emergency because of the possibility of ectopic pregnancy.