Missing your period some months usually means your body isn’t ovulating on a regular schedule. A normal menstrual cycle repeats every 21 to 35 days, so if yours frequently stretches beyond that window or skips entirely, something is interfering with the hormonal chain reaction that triggers ovulation each month. The causes range from everyday factors like stress and undereating to medical conditions that need treatment.
How a Regular Cycle Works
Your period depends on a precise relay of hormonal signals between your brain and your ovaries. A region of the brain called the hypothalamus releases a master hormone that tells the pituitary gland to send two more hormones (LH and FSH) to the ovaries. Those hormones trigger an egg to mature and release. After ovulation, a rise and then drop in estrogen and progesterone is what causes the uterine lining to shed. If any link in that chain gets disrupted, ovulation doesn’t happen on schedule, and your period either arrives late or not at all.
Stress and Your Reproductive Hormones
Chronic stress is one of the most common reasons periods become irregular, and the biology behind it is straightforward. When you’re under sustained physical or emotional stress, your adrenal glands pump out cortisol. Cortisol directly suppresses the brain’s production of the master reproductive hormone (GnRH), which in turn lowers LH and FSH. Without adequate levels of those hormones, the ovaries don’t get the signal to release an egg.
Researchers at UC Berkeley found that stress hormones hit the reproductive system in two ways at once: they suppress the hormone that drives ovulation while simultaneously boosting a separate hormone that further inhibits it. The result can be a skipped period during a stressful month, or months of irregular cycles if the stress doesn’t let up. This pattern is sometimes called functional hypothalamic amenorrhea, and it’s reversible once the underlying stressor is addressed.
Undereating, Overexercising, or Both
Your body treats reproduction as optional when energy is scarce. If you’re not eating enough to support your activity level, whether from intentional dieting, disordered eating, or simply not realizing how much fuel you need, the same hypothalamic shutdown described above can occur. Low calorie intake, high energy demands, low body fat percentage, and physical stress all contribute to hormonal changes that delay or stop periods.
This is especially common in athletes and is part of a condition called Relative Energy Deficiency in Sport (REDs). Guidelines suggest athletes may need at least 30 to 45 calories per kilogram of fat-free body mass per day to maintain normal hormonal function. Falling below that threshold regularly can cause periods to disappear for months. The fix is increasing food intake relative to exercise, not simply exercising less.
Polycystic Ovary Syndrome (PCOS)
PCOS is the most common hormonal disorder in people of reproductive age and a leading cause of irregular periods. It involves a combination of elevated androgens (hormones like testosterone that are typically higher in males), irregular or absent ovulation, and sometimes cysts visible on the ovaries via ultrasound. A diagnosis requires at least two of those three features after other causes have been ruled out.
Signs that point toward PCOS include persistent acne, hair growth on the face or chest, thinning hair on the scalp, and cycles that regularly stretch beyond 35 days. Blood tests often reveal elevated testosterone or a skewed ratio of LH to FSH. When that ratio is greater than 2 to 1, PCOS becomes likely. The condition doesn’t go away on its own, but management with lifestyle changes, and sometimes medication, can restore more regular cycles and reduce symptoms.
Thyroid Problems
Both an underactive and overactive thyroid can throw off your cycle. Your thyroid hormones interact directly with your ovaries and influence levels of several reproductive hormones. An underactive thyroid (hypothyroidism) is the more common culprit. It raises levels of a brain hormone called TRH, which in turn increases prolactin. Elevated prolactin suppresses ovulation, and without ovulation, your period either becomes very irregular or stops.
Thyroid issues are easy to screen for with a simple blood test measuring TSH. If your periods have become irregular alongside symptoms like unusual fatigue, unexplained weight changes, feeling cold all the time, or changes in your skin and hair, a thyroid problem is worth investigating.
Hormonal Birth Control
If you’re on hormonal contraception, your missing periods may be an expected side effect rather than a problem. Progestin-based methods in particular thin the uterine lining so much that there’s very little to shed. About 20% of people using a hormonal IUD stop getting periods entirely within the first year. Implants, injections, and some oral contraceptives can have a similar effect.
Periods can also take several months to return to a regular pattern after you stop hormonal birth control. This is normal and doesn’t indicate a fertility issue, though cycles that haven’t returned after three months off contraception are worth discussing with a provider.
High Prolactin Levels
Prolactin is the hormone responsible for milk production, and elevated levels outside of breastfeeding can suppress ovulation. This is why breastfeeding itself often delays the return of periods postpartum. Outside of pregnancy and nursing, high prolactin can be caused by certain medications (especially some psychiatric medications), an underactive thyroid, or a small benign growth on the pituitary gland. If blood work shows elevated prolactin, the underlying cause is usually treatable.
Perimenopause
If you’re in your late 30s or 40s, irregular periods may be an early sign of perimenopause. Estrogen levels begin to fluctuate well before menopause itself, sometimes a full decade earlier. During this transition, you might have a normal 28-day cycle one month and then wait 45 or 60 days for the next one. Flow can be heavier or lighter than usual, and the unpredictability tends to increase as menopause gets closer.
Perimenopause is a normal biological process, not a disorder. But it can look identical to other causes of irregular periods, so if you’re under 40 and experiencing these changes, it’s worth getting hormone levels checked to rule out premature ovarian changes or other conditions.
What Doctors Look For
If your previously regular periods have been absent for three months, or your previously irregular periods have been absent for six months, that meets the clinical threshold for evaluation. Cycles consistently longer than 35 days also warrant a look. Periods that suddenly stop for more than 90 days (when you’re not pregnant) are a clear signal to get checked.
The workup is usually straightforward. A standard blood panel checks thyroid-stimulating hormone, cortisol, prolactin, FSH, testosterone, and DHEA. These tests can quickly narrow down whether the issue is thyroid-related, stress-driven, linked to elevated androgens (pointing toward PCOS), or caused by high prolactin. A pregnancy test is always part of the initial evaluation, regardless of whether you think pregnancy is possible. Depending on results, an ultrasound of the ovaries or additional hormone tests may follow.
Most causes of irregular periods are treatable or manageable once identified. The specific approach depends entirely on the underlying cause: restoring energy balance, managing stress, correcting a thyroid imbalance, or addressing a hormonal condition like PCOS each require different strategies, but all can lead to more predictable cycles.