Unexpected bleeding between periods, or spotting that shows up with no obvious pattern, is one of the most common gynecological complaints. The causes range from completely harmless hormonal shifts to conditions that need treatment, so the answer depends heavily on your age, whether you’re on any medications, and how heavy the bleeding is. Here’s a breakdown of the most likely explanations and what each one actually looks like.
Hormonal Birth Control Is the Most Common Culprit
If you’re on hormonal contraception, that’s the first place to look. Unscheduled bleeding affects roughly 10 to 18% of people per cycle on combined hormonal birth control, regardless of whether it’s a pill, patch, or ring. This breakthrough bleeding is especially common during the first three to four months after starting a new method or switching doses. It doesn’t mean your contraception has failed, and it typically settles down on its own within a few cycles.
IUDs, implants, and progestin-only pills can also cause irregular spotting, sometimes for longer stretches. If you recently started, stopped, or changed a hormonal method, that’s very likely your answer. Missing pills or taking them at inconsistent times makes breakthrough bleeding even more likely.
Ovulation and Hormonal Fluctuations
Even without birth control, your hormones don’t always follow a perfect script. A brief dip in estrogen around ovulation (roughly mid-cycle) can trigger light spotting for a day or two. This is normal and happens to many people occasionally without ever becoming a pattern.
More persistent irregular bleeding often points to ovulatory dysfunction, where your body doesn’t release an egg consistently. Polycystic ovary syndrome (PCOS) is one of the most common reasons for this, but thyroid problems can also disrupt the cycle. An overactive thyroid raises levels of a protein that binds to estrogen, which changes how much free estrogen is circulating and can throw off your cycle timing. An underactive thyroid tends to cause heavier, more prolonged periods. Either way, thyroid-related bleeding usually comes with other clues like fatigue, weight changes, or feeling unusually hot or cold.
Stress and Its Effect on Your Cycle
Chronic stress raises cortisol levels, and cortisol directly interferes with the hormonal signaling chain that controls your menstrual cycle. Specifically, elevated cortisol suppresses the signals your brain sends to your ovaries, which can delay or prevent ovulation. When ovulation is disrupted, the lining of your uterus doesn’t shed on a predictable schedule, leading to random spotting or periods that show up early, late, or heavier than usual. Major life upheavals, sleep deprivation, sudden changes in exercise, or rapid weight loss can all trigger this cascade.
Polyps and Fibroids
These are two of the most common structural causes of unexpected bleeding. Polyps are small tissue growths on the uterine lining, while fibroids are benign muscle tumors that grow in or on the uterus. Both can cause spotting between periods, heavy periods, and bleeding after menopause. Many are small enough to cause no symptoms at all and are discovered incidentally during imaging for something else.
The peak incidence of endometrial polyps hits between ages 40 and 44, though they can develop earlier. Fibroids are extremely common as well, particularly in the late 30s and 40s. When either grows large enough or sits in the right location (especially just beneath the uterine lining), they can cause bleeding that seems random because it doesn’t follow your usual cycle. Small polyps and fibroids sometimes resolve on their own, but larger or symptomatic ones may need treatment to control bleeding.
Infections and Cervical Causes
Sexually transmitted infections, particularly chlamydia and gonorrhea, can cause spotting or bleeding between periods, especially after sex. These infections inflame the cervix, making it more fragile and prone to bleeding on contact. Pelvic inflammatory disease, which can develop from untreated STIs, causes deeper pelvic pain along with irregular bleeding. Many chlamydia infections produce no symptoms at all, so spotting may be the only early sign.
Cervical irritation from causes other than infection, like a cervical ectropion (where cells from the inner cervical canal appear on the outer surface), can also cause post-sex spotting. This is common during pregnancy and in people on hormonal contraception and is generally harmless, though it can be alarming.
Your Age Matters
The most likely explanation for random bleeding shifts considerably depending on where you are in your reproductive life.
In your teens and early 20s, the hormonal system controlling your cycle is still maturing. Irregular cycles and occasional spotting are common and often resolve without treatment. Bleeding disorders like von Willebrand disease are also worth considering in this age group, particularly if your periods have always been very heavy from the start.
In your 30s, structural causes like polyps and fibroids become more relevant, alongside hormonal factors and pregnancy-related bleeding.
In your 40s and early 50s, perimenopause is the leading cause of erratic bleeding. This transition typically begins in the early 40s and can last several years. As ovarian function declines, cycles become unpredictable: shorter, longer, heavier, lighter, or with spotting in between. This is a normal part of the transition, but because the risk of endometrial polyps, hyperplasia, and cancer also increases with age, new or worsening irregular bleeding in this window should be evaluated.
After menopause, any bleeding is considered abnormal. About 9% of people who experience postmenopausal bleeding are diagnosed with endometrial cancer. That means the vast majority of cases (roughly 91%) have a benign explanation, often atrophy of the uterine lining or polyps. Still, postmenopausal bleeding always warrants a medical evaluation to rule out the serious causes.
Medications Beyond Birth Control
Blood thinners (anticoagulants) are a well-known cause of heavier or more frequent bleeding. Tamoxifen, used in breast cancer treatment, can stimulate the uterine lining and cause spotting. Even supplements like high-dose fish oil or certain herbal products with blood-thinning properties can contribute. If your bleeding started after beginning a new medication, that connection is worth raising with your provider.
What the Evaluation Looks Like
If your bleeding is persistent, getting heavier, or doesn’t have an obvious explanation like a new birth control method, a provider will typically start with a few straightforward steps. Blood work checks for pregnancy, thyroid function, and signs of a bleeding disorder. An ultrasound (usually transvaginal) looks for structural problems like polyps, fibroids, or thickening of the uterine lining. If the ultrasound raises concerns, or if you have risk factors for endometrial problems, an endometrial biopsy may follow. This is a quick in-office procedure that samples the uterine lining to check for abnormal cells.
For younger people with a clear explanation like new birth control or an irregular cycle with no other symptoms, watchful waiting for a few months is often reasonable.
When Bleeding Needs Urgent Attention
Most random spotting doesn’t qualify as an emergency, but heavy bleeding does. The threshold to watch for: soaking through one or more pads or tampons every hour for more than two hours in a row. Bleeding that heavy, especially if accompanied by dizziness, lightheadedness, or a racing heart, needs immediate medical attention. Bleeding during a confirmed or possible pregnancy also warrants urgent evaluation, as it can signal a miscarriage or ectopic pregnancy.