Vaginal bleeding has a wide range of causes, from completely harmless hormonal shifts to conditions that need prompt treatment. The cause depends heavily on your age, whether you could be pregnant, and the pattern of bleeding you’re experiencing. Understanding the most common reasons can help you recognize what’s routine and what deserves medical attention.
Pregnancy-Related Causes
If there’s any chance you could be pregnant, that’s the first thing to rule out. Miscarriage is the most common cause of bleeding in early pregnancy, occurring in about 10 to 15 percent of all pregnancies. It typically starts as spotting and progresses to heavier bleeding with cramping. Not all early pregnancy bleeding means miscarriage, though. Threatened miscarriage, where the placenta partially detaches but the pregnancy continues, can cause spotting or light bleeding without pain.
Ectopic pregnancy, where a fertilized egg implants outside the uterus (usually in a fallopian tube), occurs in about 1 percent of pregnancies. The bleeding is often scanty and brownish rather than bright red. This is a medical emergency because a ruptured ectopic pregnancy can cause life-threatening internal bleeding. Any bleeding in early pregnancy paired with sharp abdominal pain or dizziness warrants immediate care.
Hormonal Imbalances
Your menstrual cycle relies on a carefully timed balance between estrogen and progesterone. When that balance is off, the uterine lining can build up too thick or shed unpredictably, leading to irregular or heavy bleeding. Several conditions disrupt this balance.
Polycystic ovary syndrome (PCOS) is one of the most common. It causes irregular or absent ovulation, which means progesterone levels stay low while estrogen continues stimulating the uterine lining. The result is often long gaps between periods followed by very heavy bleeding when a period finally arrives. Thyroid disorders and conditions affecting the brain’s hormonal signaling centers can cause similar patterns.
During perimenopause, the years leading up to menopause, hormone levels fluctuate dramatically. Periods may come closer together, further apart, or vary widely in heaviness from month to month. When estrogen dominates without enough progesterone to balance it, the uterine lining can thicken excessively, a condition called endometrial hyperplasia. This thickening causes unpredictable, sometimes heavy bleeding and, if left untreated, carries a small risk of progressing toward cancer.
Fibroids, Polyps, and Other Structural Causes
Physical growths in or on the uterus are extremely common causes of abnormal bleeding. Fibroids are benign muscle tumors that develop in the uterine wall. They can range from pea-sized to large enough to distort the shape of the uterus. Fibroids that press into the uterine cavity tend to cause the heaviest bleeding, while those growing on the outer surface may cause no bleeding symptoms at all. Many people with fibroids never know they have them.
Uterine polyps are soft growths that attach to the inner wall of the uterus and are sensitive to estrogen, meaning they grow in response to it. They commonly cause bleeding between periods, unusually heavy periods, and bleeding after menopause. Most polyps are benign, but a small percentage carry malignancy risk, particularly in postmenopausal women.
Adenomyosis is a lesser-known condition where tissue that normally lines the uterus grows into the muscular wall instead. It often causes heavy, painful, prolonged periods along with a feeling of pelvic pressure. The uterus itself may become enlarged.
Infections and Inflammation
Sexually transmitted infections, particularly chlamydia and gonorrhea, can inflame the cervix and make it bleed easily, especially during or after sex. If these infections spread to the uterus, fallopian tubes, or ovaries, they cause pelvic inflammatory disease (PID). PID symptoms include bleeding between periods, pain during sex, and pelvic pain, though some people have very mild or no symptoms at all.
Chronic inflammation of the uterine lining (endometritis) can also cause irregular spotting. One study found that 29 percent of women on birth control pills who developed new abnormal bleeding had undiagnosed chlamydia infection or chronic endometritis, highlighting how easily infections can be overlooked as a bleeding cause.
Medications and Contraceptives
Hormonal birth control is one of the most frequent medication-related causes of unexpected bleeding. Up to 30 percent of women experience abnormal bleeding in their first month on combination birth control pills, and even missing a single pill can trigger breakthrough bleeding. The bleeding happens because hormonal contraceptives thin the uterine lining, which can become fragile and shed at unpredictable times.
Progestin-only methods cause even more bleeding irregularity. Around 70 percent of people using contraceptive injections and up to 80 percent of those using implants experience unpredictable bleeding episodes during the first year. This typically improves over time but can be a significant nuisance early on. Smoking makes breakthrough bleeding more likely if you’re on hormonal contraceptives, with smokers about 47 percent more likely to experience it than nonsmokers.
Blood-thinning medications can also increase menstrual bleeding or cause spotting between periods. Certain breast cancer treatments that affect estrogen levels are another known cause.
Vaginal and Cervical Sources
Not all vaginal bleeding originates from the uterus. The vagina and cervix themselves can be sources, and the distinction matters for figuring out the cause.
Vaginal atrophy is one of the most common non-uterine sources, particularly during and after menopause. As estrogen levels drop, the vaginal lining becomes thin, dry, and fragile with reduced blood flow. This thinned tissue can bleed easily, especially during sex. Classic signs include vaginal dryness, redness, and a loss of stretchiness. Atrophy can also develop during breastfeeding or after surgical removal of the ovaries.
Physical trauma from sex, foreign objects, or injury can cause bleeding from the vaginal walls or cervix. Cervical ectropion, where the delicate cells from inside the cervical canal are present on the outer surface, can cause spotting after sex and is particularly common in younger women or those taking hormonal contraceptives.
Bleeding After Menopause
Any bleeding after menopause (defined as 12 consecutive months without a period) needs evaluation, but the news is reassuring more often than not. A large analysis of over 40,000 patients found that about 9 percent of women with postmenopausal bleeding were diagnosed with endometrial cancer. That means roughly 91 percent had a non-cancerous cause, such as vaginal atrophy, polyps, or endometrial thinning.
The relationship works the other way too: about 90 percent of women who do develop endometrial cancer experience postmenopausal bleeding as a symptom. This is why the symptom is taken seriously even though cancer is not the most likely explanation. Risk factors for endometrial cancer include prolonged estrogen exposure without progesterone, such as from obesity, late menopause, or certain hormone therapies.
Bleeding Disorders
Systemic conditions that affect blood clotting can cause unusually heavy periods, sometimes starting from the very first menstrual cycle. Von Willebrand disease is the most common inherited bleeding disorder and is a frequent cause of heavy menstrual bleeding in adolescents and young adults. If your periods have always been extremely heavy, or if you also bruise easily, bleed heavily from cuts, or have had bleeding problems during dental work, a clotting disorder may be worth investigating.
How Heavy Is Too Heavy
Normal periods last up to seven days and require a pad or tampon change every few hours. Bleeding is considered heavy when you need to change your pad or tampon more often than every two hours, or when you’re passing large clots. If you’re soaking through one or more pads or tampons every hour for several consecutive hours, that level of bleeding needs prompt medical attention. Periods lasting longer than seven days also fall outside the normal range and are worth discussing with a provider, especially if the pattern is new for you.