Bleeding between periods is common, affecting up to 35% of women of reproductive age at some point. Most of the time it’s harmless, caused by something as simple as a temporary hormone shift. But it can also signal an infection, a structural change in the uterus, or a side effect of birth control. Understanding the most likely causes helps you figure out whether what you’re experiencing is worth a phone call to your doctor or just your body doing something normal.
Spotting vs. Heavier Bleeding
The first thing to sort out is how much blood you’re actually seeing. Spotting produces a small amount of blood, usually just a few drops that show up on underwear or when you wipe. It doesn’t require a pad or tampon. If the bleeding is heavy enough that you need period products, that’s closer to breakthrough bleeding, and the possible causes shift accordingly.
Color matters too. Spotting between periods is often light pink or brown rather than the bright or dark red of a full period. Brown blood is simply older blood that took longer to leave the uterus. Light pink blood mixed with cervical fluid is typical of ovulation spotting. Bright red bleeding that lasts more than a day or two outside your period is worth paying closer attention to.
Ovulation Spotting
The most common and least concerning cause of mid-cycle bleeding is ovulation. In the days leading up to ovulation, estrogen levels climb steadily. Once the egg is released, estrogen dips and progesterone starts to rise. That sudden shift can cause the uterine lining to shed just slightly, producing light spotting. It typically happens around day 14 of a 28-day cycle, though your timing may vary.
Ovulation spotting usually lasts one to two days and stops on its own. You might also notice mild cramping on one side of your lower abdomen, sometimes called mittelschmerz. If you’re tracking your cycle and the spotting lines up with your expected ovulation window, this is very likely what’s happening. No treatment is needed.
Hormonal Birth Control
Birth control is one of the most frequent triggers for between-period bleeding, especially in the first three to six months after starting a new method. The pill, the patch, hormonal IUDs, implants, and the shot can all cause irregular spotting as your body adjusts to a different hormone balance.
Missing a pill or taking it at inconsistent times is a particularly common cause. When hormone levels drop even briefly, the uterine lining can become unstable enough to shed a little. If you’ve recently switched methods or missed doses, that’s the most likely explanation. The spotting usually resolves once your body adapts or once you become more consistent with timing. Persistent breakthrough bleeding beyond three months is worth discussing with your provider, since switching formulations often fixes the problem.
Cervical Infections
Infections of the cervix, called cervicitis, can make cervical tissue fragile and prone to bleeding. The most common culprits are sexually transmitted infections: chlamydia, gonorrhea, trichomoniasis, genital herpes, and HPV. Bacterial vaginosis, though not sexually transmitted, can also irritate the cervix enough to cause spotting.
The hallmark pattern is bleeding after sex or between periods, sometimes accompanied by unusual discharge, a change in discharge color or smell, or pelvic discomfort. Chlamydia and gonorrhea are especially worth considering because they often cause no other symptoms at all, meaning spotting may be your only clue. If there’s any chance of a new or untreated STI, testing is straightforward and treatment is usually a short course of antibiotics.
Uterine Polyps and Fibroids
Structural growths inside the uterus are another well-established cause. Uterine polyps are small, soft growths that form when cells in the uterine lining overgrow. They’re estrogen-sensitive, meaning they grow in response to your body’s own estrogen. Polyps can cause irregular bleeding between periods, unusually heavy periods, or both. They’re more common as you get older but can occur at any age.
Fibroids are noncancerous muscle growths in the uterine wall. Depending on their size and location, they can distort the uterine lining and cause bleeding at unpredictable times. Submucosal fibroids, the type that grows into the uterine cavity, are the most likely to cause irregular bleeding. Both polyps and fibroids are typically found through ultrasound. Many are small and need no treatment, but those causing persistent bleeding can be removed through minimally invasive procedures.
Perimenopause
If you’re in your 40s (or sometimes late 30s) and your cycles have started to feel unpredictable, perimenopause is a strong possibility. During this transition, estrogen and progesterone rise and fall erratically rather than following the smooth pattern of earlier reproductive years. You may ovulate some months and skip others. When ovulation doesn’t happen, progesterone stays low, and the uterine lining can shed at odd times or build up and then release heavily.
Periods during perimenopause may come closer together, further apart, lighter, or heavier than what you’re used to. Spotting between periods fits right into this pattern. The transition can last anywhere from a few years to a decade before periods stop entirely. While irregular bleeding in perimenopause is usually hormonal, new or worsening bleeding after age 45 is still worth evaluating because the risk of endometrial changes increases with age.
Less Common but Important Causes
Pregnancy should always be on the list if there’s any chance you could be pregnant. Implantation bleeding occurs when a fertilized egg attaches to the uterine lining, typically six to twelve days after conception. It’s usually very light and brief. Ectopic pregnancy and early miscarriage can also present as unexpected bleeding, sometimes with cramping or one-sided pain.
Thyroid disorders, particularly an underactive thyroid, can disrupt the hormonal signals that regulate your cycle and cause bleeding at irregular intervals. Polycystic ovary syndrome (PCOS) is another hormonal condition that leads to infrequent ovulation and unpredictable bleeding patterns. In rare cases, persistent intermenstrual bleeding can be a sign of cervical or endometrial changes that require a biopsy to evaluate, particularly for women with risk factors like obesity, a history of irregular periods, or age over 45.
What Evaluation Looks Like
If the bleeding is a one-time event that was light and resolved quickly, it’s reasonable to simply note when it happened and watch for a pattern. If it recurs over multiple cycles, is heavy, happens after sex, or comes with pain or unusual discharge, an evaluation will help narrow down the cause.
A typical workup starts with a medical history and pelvic exam. Your provider will likely ask about your cycle length, how much bleeding you’re seeing, whether it follows sex, and what birth control you use. Depending on your age and risk factors, the next steps could include STI testing, blood work to check hormone levels or thyroid function, and a transvaginal ultrasound to look at the uterine lining and check for polyps or fibroids. If the ultrasound suggests something abnormal, a closer look with a specialized procedure called sonohysterography or hysteroscopy may follow. For women at increased risk of endometrial changes, an endometrial biopsy, a quick in-office procedure, can rule out more serious causes.
Tracking your bleeding before your appointment makes the evaluation more efficient. Note the dates, how much blood you see, what color it is, and whether anything seems to trigger it. Even a few months of data gives your provider a much clearer picture than a general description.