Irregular bleeding that starts, stops, and starts again usually signals that something is disrupting the normal hormonal cycle that controls your uterine lining, or that a structural change inside the uterus is causing it to shed unpredictably. The causes range from completely benign (a new birth control pill, stress, a shifting cycle) to things that need medical attention (fibroids, infections, or rarely, precancerous changes). Understanding the most common reasons can help you figure out what might apply to you.
How Normal Bleeding Works
Your uterine lining builds up each month in response to estrogen, then stabilizes under progesterone after ovulation. When progesterone drops at the end of the cycle, the lining sheds in a predictable window. On-and-off bleeding happens when that hormonal sequence gets interrupted, when the lining grows unevenly, or when something physically irritates the uterine or cervical tissue. Doctors group the causes into two broad buckets: structural problems you can see on imaging (polyps, fibroids, tissue overgrowth) and non-structural problems involving hormones, clotting, medications, or infections.
Hormonal Shifts and Ovulation Problems
The most common reason for intermittent bleeding in reproductive-age women is ovulatory dysfunction. When you don’t ovulate in a given cycle, your body never produces the surge of progesterone that normally stabilizes the lining. Without that signal, the lining builds unevenly under estrogen alone and sheds in patches, causing light bleeding that comes and goes rather than one clean period. Conditions like polycystic ovary syndrome (PCOS), thyroid disorders, and significant stress or weight changes all make irregular ovulation more likely.
Research on premenopausal women shows that thyroid hormone levels influence the reproductive hormones that govern your cycle, even when thyroid function falls within the “normal” range. Women with lower thyroid activity tend to have shorter cycles and lower progesterone, while those with an overactive thyroid are more likely to have very light or infrequent periods. The rate at which progesterone drops at the end of your cycle also matters. A slower, more gradual decline is associated with spotting before your period begins, which can make it feel like bleeding drags on or restarts.
Birth Control and Medications
If you recently started, switched, or missed doses of hormonal birth control, breakthrough bleeding is the most likely explanation. It’s extremely common in the first month of a new pill, with rates up to four and a half times higher than baseline. For most combined oral contraceptives, bleeding settles down within three to four months. Pills with lower estrogen doses (15 or 20 micrograms) can take significantly longer to produce a regular pattern than standard-dose pills (30 micrograms).
IUDs, implants, and injections each have their own bleeding profiles. Hormonal IUDs often cause irregular spotting for the first three to six months. The copper IUD can make periods heavier and longer. Beyond contraception, blood thinners, certain antidepressants, and the breast cancer drug tamoxifen are all known to trigger uterine bleeding as a side effect.
Fibroids and Polyps
Fibroids are benign muscle growths in the uterine wall. They range from the size of a grain of rice to as large as a melon, and they can appear alone or in clusters. Only about one third are large enough for a provider to feel during a pelvic exam, so many go undiagnosed. Fibroids that grow into the uterine cavity are the ones most likely to cause bleeding between periods, heavy periods, or both. Over time, repeated blood loss from fibroids can lead to iron-deficiency anemia.
Endometrial polyps are small, finger-like tissue growths on the uterine lining. They’re especially common in women aged 40 to 44 and are one of the top structural causes of on-and-off bleeding. Polyps may bleed when irritated or when hormonal shifts cause them to partially shed. Both fibroids and polyps are typically found on ultrasound or during a procedure called hysteroscopy, where a thin camera is guided through the cervix to view the inside of the uterus.
Infections and STIs
Bleeding between periods can be a sign of infection, particularly pelvic inflammatory disease (PID). PID is most often caused by untreated chlamydia or gonorrhea that spreads upward from the cervix into the uterus and fallopian tubes. Along with intermenstrual bleeding, symptoms include lower abdominal pain, unusual or foul-smelling discharge, pain during sex, and burning with urination. Some women with PID have very mild symptoms and don’t realize they have an infection until bleeding prompts a visit. Cervicitis, an inflammation of the cervix from infection or irritation, can also cause spotting, especially after intercourse.
Perimenopause
If you’re in your late thirties or forties, erratic bleeding may simply be your body entering perimenopause, the transition period before menopause that typically begins in the early forties and can last several years. During this time, ovulation becomes less predictable, and cycles can swing between short and long, heavy and light. You might skip a period entirely, then bleed for two weeks straight the following month. The hormonal swings of perimenopause also increase the likelihood of developing polyps, fibroids, and adenomyosis (a condition where uterine lining tissue grows into the muscular wall of the uterus, causing pain and heavy or prolonged bleeding). The incidence of all three rises through the fourth decade of life.
When Bleeding May Signal Something Serious
Most intermittent bleeding turns out to have a manageable cause, but certain patterns warrant prompt evaluation. Any bleeding after menopause is considered abnormal. About 9% of postmenopausal women who see a doctor for bleeding are eventually diagnosed with endometrial cancer, and 90% of women with endometrial cancer reported abnormal bleeding before their diagnosis. When caught early, the five-year survival rate is 95%. When caught late, after the cancer has spread, that number drops to between 16% and 45%. That gap makes early evaluation genuinely important.
In premenopausal women, bleeding that soaks through a pad or tampon every hour for several hours, bleeding that lasts longer than seven days per cycle, or spotting that persists for more than three months on a new contraceptive all deserve investigation. Bleeding after sex that happens repeatedly is another pattern to take seriously, as it can point to cervical issues, polyps, or infection.
How Doctors Figure Out the Cause
Evaluation usually starts with a pelvic exam and blood work. A complete blood count checks for anemia and signs of infection. Depending on your age and symptoms, your provider may also check thyroid function, clotting factors, or pregnancy status. The next step is often a transvaginal ultrasound, which uses sound waves to create images of the uterus and ovaries and can reveal fibroids, polyps, or thickened endometrial tissue.
If the ultrasound is inconclusive or shows something that needs a closer look, a hysteroscopy allows the doctor to view the uterine cavity directly through a thin scope. An endometrial biopsy, where a small sample of the uterine lining is collected and examined under a microscope, is used to check for precancerous changes or cancer. This is especially relevant for women over 40 or those with risk factors like prolonged irregular cycles, obesity, or a family history of uterine cancer.
Common Causes by Age Group
- Teens and early twenties: Immature hormonal cycles that haven’t settled into a regular ovulation pattern, bleeding disorders like von Willebrand disease (a common clotting condition that often surfaces with heavy periods in adolescence), and STIs.
- Mid-twenties to late thirties: Hormonal contraception side effects, PCOS, polyps, early fibroids, pregnancy-related causes, and infections.
- Forties and beyond: Perimenopausal hormonal shifts, growing fibroids, polyps, adenomyosis, thyroid dysfunction, and endometrial hyperplasia or cancer. The risk of structural and precancerous causes rises steadily with age.
Tracking your bleeding pattern for a few cycles, noting when it starts and stops, how heavy it is, and whether it follows sex or exercise, gives your provider the clearest picture to work with. Even a simple notes app or period-tracking app entry can make the diagnostic process faster and more accurate.