Bleeding between periods has a wide range of causes, from completely harmless hormonal shifts to conditions that need treatment. In most cases, the spotting turns out to be something manageable, but the pattern, amount, and timing of the bleeding all matter when figuring out what’s behind it.
Ovulation Spotting
One of the most common and least concerning causes is ovulation spotting. In the days leading up to ovulation, your estrogen levels climb steadily. Once the egg is released, estrogen dips and progesterone takes over. That hormonal shift can trigger light bleeding or spotting, typically around the midpoint of your cycle (roughly day 14 in a 28-day cycle). It’s fairly common, usually lasts a day or two, and the blood is often light pink or brown rather than the bright red of a full period. If you notice a pattern of light spotting about two weeks before your period each month, ovulation is the likely explanation.
Hormonal Birth Control
Starting or switching a hormonal contraceptive is one of the most frequent triggers for breakthrough bleeding. Your body needs time to adjust to the new hormone levels, and spotting during that transition is expected, not a sign that something is wrong or that the method isn’t working.
The adjustment timeline depends on the type of contraceptive. With an IUD, spotting and irregular bleeding in the first few months after placement is normal and typically resolves within two to six months. The implant works a bit differently: whatever bleeding pattern you have in the first three months tends to be your pattern going forward. If breakthrough bleeding persists beyond those windows, or suddenly appears after months of no issues, it’s worth checking in with your provider. Missing pills, taking them at inconsistent times, or interactions with other medications can also cause spotting on the combination pill.
Uterine Polyps
Uterine polyps are soft growths that form on the inner wall of the uterus when the lining overgrows. They range from the size of a sesame seed to as large as a golf ball, and they’re fueled by estrogen. Bleeding between periods is one of their hallmark symptoms, along with unpredictable periods that vary in length and heaviness.
Polyps are most common in people approaching or past menopause, but younger people develop them too. Your risk goes up with obesity, use of the breast cancer drug tamoxifen, or hormone replacement therapy. Most polyps are benign, though a small percentage can contain precancerous or cancerous cells, which is why doctors generally recommend removing larger ones or those causing symptoms. Removal is usually a straightforward outpatient procedure.
Fibroids
Fibroids are noncancerous muscular growths in or on the uterus. They’re extremely common, particularly in people over 30, and many cause no symptoms at all. But fibroids that grow into the uterine cavity or distort its shape can cause heavy periods, bleeding between periods, pelvic pressure, and pain. Benign growths like fibroids and polyps both become more common with age, which is one reason intermenstrual bleeding increases during the perimenopausal years.
Infections and Cervicitis
Sexually transmitted infections, particularly chlamydia, gonorrhea, trichomoniasis, and genital herpes, can inflame the cervix, a condition called cervicitis. When the cervix is irritated and swollen, it bleeds more easily, including between periods or after sex. Some people with cervicitis also notice unusual discharge or pain during urination, but others have no symptoms beyond the spotting itself.
Because chlamydia and gonorrhea often produce few obvious symptoms, unexplained intermenstrual bleeding in someone who is sexually active is a good reason to get tested. These infections are easily treated with antibiotics, but left untreated they can spread to the uterus and fallopian tubes and cause more serious complications.
Perimenopause
If you’re in your 40s (or sometimes late 30s) and your cycle has started behaving unpredictably, perimenopause may be the reason. During this transition, ovarian function becomes less stable. Estrogen and progesterone levels fluctuate more dramatically from month to month, which can lead to shorter cycles, longer cycles, skipped periods, heavier bleeding, and spotting between periods. This kind of irregular bleeding is often part of normal physiology during perimenopause, not a sign of disease.
That said, the perimenopausal years also overlap with the age when polyps, fibroids, and less common conditions like endometrial hyperplasia become more prevalent. So while irregular bleeding during this stage is expected, new or worsening patterns still deserve evaluation, especially any bleeding that occurs after you’ve gone 12 months without a period (which would classify as postmenopausal bleeding and always warrants investigation).
Blood Thinners and Medications
If you take anticoagulant (blood-thinning) medications for a blood clot or heart condition, they can significantly affect your menstrual bleeding. Research from the American Society of Hematology found that 66% of women on blood thinners experienced abnormally heavy uterine bleeding at some point during a follow-up period. Despite how common this side effect is, there are currently no standard guidelines for monitoring menstrual changes after starting these medications. If you’ve recently begun a blood thinner and notice new spotting or heavier bleeding, bring it up with your prescriber so you can weigh the risks together.
Certain supplements with blood-thinning properties, like high-dose fish oil, vitamin E, or ginkgo biloba, can have a similar, milder effect.
Less Common but Serious Causes
Endometrial cancer and precancerous changes to the uterine lining (endometrial hyperplasia) can cause intermenstrual bleeding. These conditions are far less common than the causes listed above, but the risk increases with age, obesity, a history of irregular ovulation, and use of tamoxifen. There’s no routine screening test for endometrial cancer in the general population, but abnormal vaginal bleeding is the symptom that prompts most diagnoses. People with a hereditary condition called Lynch syndrome (hereditary nonpolyposis colorectal cancer) are at higher risk and may benefit from yearly screening with transvaginal ultrasound starting as early as age 25.
Cervical cancer, ectopic pregnancy, and certain blood clotting disorders are other uncommon but important causes. Any bleeding during a confirmed pregnancy should be evaluated promptly.
What to Expect at the Doctor
If your intermenstrual bleeding is more than occasional light spotting, your doctor will likely start with a detailed history: when the bleeding happens, how much there is, whether it follows sex, and what medications or contraceptives you use. Blood tests can check for pregnancy, thyroid problems, and clotting issues. A pelvic exam may reveal cervical inflammation or visible growths.
If structural causes like polyps or fibroids are suspected, the next step is usually a transvaginal ultrasound, a painless imaging scan that gives a clear picture of the uterine lining and cavity. If the ultrasound isn’t conclusive, additional procedures like a saline-infusion sonogram or hysteroscopy (a thin camera inserted through the cervix) can identify small growths that standard imaging might miss. For people at increased risk of endometrial hyperplasia or cancer, an endometrial biopsy, where a small tissue sample is taken from the uterine lining, may be recommended. MRI is occasionally used when other methods aren’t feasible or conclusive.
Signs That Need Urgent Attention
Most intermenstrual bleeding isn’t an emergency, but certain combinations of symptoms are. 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, seek emergency care immediately. These signs together can indicate significant blood loss that needs rapid treatment.