Bleeding between periods, sometimes called spotting or intermenstrual bleeding, is common and usually has a straightforward explanation. Hormonal shifts, birth control, infections, pregnancy, and structural changes in the uterus can all cause it. While most causes are not dangerous, some patterns of bleeding do need medical attention, so understanding what’s behind yours matters.
Hormonal Birth Control Is the Most Common Cause
If you recently started or switched a hormonal contraceptive, that’s the most likely explanation. About 30% of women experience unexpected bleeding in the first month of taking combination birth control pills. The good news is that this drops significantly by the third month as your body adjusts to the new hormone levels. During that initial window, irregular spotting is considered normal and doesn’t mean the pill isn’t working.
Injections and implants cause even more unpredictable bleeding. Up to 70% of women using contraceptive injections and 80% of those with implants experience episodes of spotting or irregular bleeding during the first year. Missing a pill or taking it at inconsistent times can also trigger breakthrough bleeding, since even small dips in hormone levels can cause the uterine lining to shed a little.
It Could Be an Early Sign of Pregnancy
Implantation bleeding happens when a fertilized egg attaches to the uterine wall, typically 10 to 14 days after ovulation. It’s one of the earliest signs of pregnancy, and it looks quite different from a period. The blood is usually pink or brown, light enough to show up as a spot in your underwear or on toilet paper when you wipe. It lasts anywhere from a few hours to a couple of days and resembles the flow of normal vaginal discharge more than menstrual bleeding.
If your bleeding is bright or dark red, heavy, or contains clots, it’s probably not implantation bleeding. A good rule of thumb: implantation bleeding might require a thin panty liner at most, but it should never soak through a pad. If there’s any chance you could be pregnant and you’re seeing light spotting around when your period would be due, a home pregnancy test is a reasonable next step.
Infections That Cause Spotting
Sexually transmitted infections like chlamydia, gonorrhea, and trichomoniasis can inflame the cervix, a condition called cervicitis. When the cervix is inflamed, its tissue becomes fragile and bleeds more easily, especially after sex. You might notice spotting along with a watery or discolored discharge, though some infections cause bleeding with no other obvious symptoms at all.
Bacterial vaginosis and other vaginal infections can also be responsible. The key clue that an infection might be involved is bleeding that happens after intercourse, since inflamed cervical tissue is particularly vulnerable to contact. If your spotting follows that pattern, or if it comes with unusual discharge, odor, or pelvic discomfort, testing for infections can identify or rule out the cause quickly.
Polyps and Fibroids
Endometrial polyps are small growths on the lining of the uterus, and they’re a surprisingly common cause of abnormal bleeding. Polyps are implicated in roughly half of all cases of abnormal uterine bleeding, and about 68% of women who have them experience irregular bleeding as their main symptom. They’re especially worth considering if your bleeding has been happening on and off for a while without an obvious trigger.
Fibroids, particularly the type that grows into the uterine cavity (submucosal fibroids), can produce the same kind of irregular bleeding. Both polyps and fibroids are noncancerous in the vast majority of cases, but they don’t always resolve on their own. An ultrasound is usually the first step to check for them. Treatment depends on size, symptoms, and whether they’re affecting your quality of life.
Hormonal Changes With Age
If you’re in your late 30s or 40s, shifting hormone levels may be behind your spotting even if menopause feels far off. Starting in the late 30s, progesterone production declines and ovulation becomes less consistent. Without enough progesterone to keep the uterine lining in check, it can build up unevenly and shed at unexpected times, causing spotting or unusually heavy periods.
Estrogen levels also become more erratic during this transition. They can drop sharply one cycle and spike higher than normal the next, which makes cycle length, flow, and timing increasingly unpredictable. This phase, called perimenopause, can last several years before periods stop entirely. Irregular bleeding during perimenopause is common, but new or worsening patterns are still worth mentioning to your doctor, since the risk of other causes like polyps and hormonal conditions also increases with age.
Other Possible Triggers
Ovulation itself causes light spotting for some women. A small dip in estrogen around the time an egg is released can trigger a day or two of pink or light brown spotting, usually about two weeks before your next period. This is harmless and tends to happen consistently once you start noticing it.
Cervical irritation from a pap smear, a recent pelvic exam, or vigorous intercourse can also cause brief spotting that resolves within a day. Significant stress, rapid weight changes, and extreme exercise can all disrupt the hormonal signals that regulate your cycle, leading to spotting or skipped periods. Thyroid disorders are another overlooked cause, since both an overactive and underactive thyroid can throw off menstrual regularity.
When Bleeding Needs Urgent Attention
Most intermenstrual spotting is light and resolves on its own or with simple treatment. But certain patterns signal something more serious. If you’re soaking through a pad or tampon every hour for more than two hours in a row, especially if you also feel dizzy, lightheaded, short of breath, or have chest pain, that’s a medical emergency.
Outside of that acute scenario, bleeding that persists for several weeks, gets progressively heavier, or happens after menopause warrants evaluation. Your doctor will likely start with a physical exam and possibly an ultrasound. For women with risk factors for endometrial conditions, such as obesity, a history of irregular cycles, or age over 45, a tissue sample from the uterine lining may be recommended to rule out more serious changes. The evaluation is typically straightforward and helps narrow down the cause so you can get the right treatment.