Amenorrhea is the absence of menstrual periods in someone of reproductive age. It comes in two forms: primary amenorrhea, where a period never starts, and secondary amenorrhea, where periods stop after they’ve already begun. The distinction matters because the causes, and what happens next, differ significantly between the two.
Primary vs. Secondary Amenorrhea
Primary amenorrhea is diagnosed when a girl hasn’t had her first period by age 15, or within three years of breast development beginning. This form is less common and usually points to a developmental or genetic cause, such as differences in reproductive anatomy or chromosomal conditions that affect how the ovaries or uterus form.
Secondary amenorrhea is far more common. It’s defined as missing periods for three or more months if your cycles were previously regular, or six or more months if they were irregular. Pregnancy is the most obvious reason periods stop, but when pregnancy isn’t the cause, something else is disrupting the hormonal chain of events that produces a monthly cycle.
How the Menstrual Cycle Gets Disrupted
A normal menstrual cycle depends on a signaling chain that starts in the brain. A region called the hypothalamus sends timed pulses of a hormone that tells the pituitary gland to release two other hormones, which in turn tell the ovaries to mature an egg and produce estrogen and progesterone. If any link in that chain breaks, periods can stop.
One of the most common disruptions is functional hypothalamic amenorrhea, where the brain essentially decides the body isn’t in a safe enough state to support reproduction. This happens when energy intake is too low relative to energy expenditure, during periods of significant psychological stress, or both. The hormone leptin, which is produced by fat cells and signals energy status to the brain, plays a central role. When leptin drops too low, the brain dials down reproductive hormones along with thyroid function and other metabolic processes. This is the mechanism behind amenorrhea in eating disorders, extreme dieting, and heavy athletic training without adequate fueling.
Common Causes
The list of potential causes is long, but a few account for the majority of cases:
- Polycystic ovary syndrome (PCOS) is one of the most frequent hormonal causes. In women with PCOS, elevated testosterone and an imbalanced ratio of certain pituitary hormones prevent regular ovulation. About 30% of women with PCOS experience amenorrhea, while another 45% have infrequent periods.
- Low energy availability from undereating, overexercising, or both suppresses the brain’s reproductive signaling. This is sometimes called the female athlete triad when it occurs alongside low bone density and disordered eating.
- Thyroid disorders can halt periods. An underactive thyroid raises levels of a pituitary hormone called TSH, which can interfere with the hormones that drive ovulation.
- High prolactin levels from a small benign pituitary growth called a prolactinoma, or as a side effect of certain psychiatric medications, can suppress menstrual cycles. Prolactin levels above 200 ng/mL almost always indicate a prolactinoma or medication effect.
- Premature ovarian insufficiency (POI) means the ovaries stop functioning normally before age 40. It affects roughly 1 in 100 women and is diagnosed when periods become irregular or absent and a blood marker called FSH rises above 25 IU/L. Unlike typical menopause, POI can sometimes be intermittent, with ovarian function flickering on and off.
- Uterine scarring (Asherman syndrome) is a structural cause where scar tissue forms inside the uterus, usually after a surgical procedure like a dilation and curettage following a miscarriage or delivery. Unlike hormonal causes, the ovaries and brain are functioning normally, but the uterine lining can’t build up and shed as it should.
What Happens During Evaluation
The first step is a pregnancy test. After that, blood work typically checks thyroid function, prolactin, and FSH levels. These three tests can quickly sort causes into broad categories. Elevated FSH suggests the ovaries aren’t responding. Elevated prolactin points toward a pituitary issue or medication side effect. Abnormal thyroid levels indicate a thyroid problem. If all three come back normal, PCOS or hypothalamic amenorrhea become the leading suspects, and your doctor will look at additional clues like body weight, exercise habits, stress levels, and signs of excess androgen (acne, excess hair growth).
Sometimes a progestin challenge is used to gather more information. You take a short course of a progesterone-like medication, and if bleeding occurs after stopping it, that confirms your body is producing enough estrogen and the uterine lining can respond to hormones. If no bleeding occurs, estrogen levels are likely very low (pointing to hypothalamic or pituitary dysfunction) or there’s a structural problem like uterine scarring. In cases where Asherman syndrome is suspected, imaging or a direct look inside the uterus with a small camera confirms the diagnosis.
Why It Matters Beyond Fertility
Missing periods isn’t just a fertility concern. Estrogen plays a critical role in maintaining bone density, and when levels stay low for months or years, bones weaken significantly. Women with amenorrhea from anorexia nervosa lose about 2.5% of bone density at the hip and spine each year. Women with exercise-related amenorrhea also show lower bone density, though typically not as severe. This bone loss can be difficult to fully reverse and increases the risk of stress fractures in the short term and osteoporosis later in life.
Low estrogen also affects cardiovascular health, brain function, and vaginal and urinary tract tissue over time. For women with premature ovarian insufficiency, these long-term effects are especially important because estrogen levels drop decades earlier than expected.
How Periods Come Back
The path to recovery depends entirely on the cause. For hypothalamic amenorrhea driven by low body weight or excessive exercise, the primary treatment is eating more and reducing training intensity. Periods typically resume within six months of reaching about 90% of a healthy body weight, though some women experience delays ranging from six months to several years after weight recovery. The variation likely depends on how long periods were absent and individual differences in how quickly the brain’s reproductive signaling restarts.
For PCOS, treatment focuses on managing the hormonal imbalance. Depending on your goals, this might involve hormonal contraceptives to regulate cycles or medications that address insulin resistance, which often drives the condition. For thyroid-related amenorrhea, treating the thyroid disorder usually restores cycles. Prolactinomas typically respond well to medication that shrinks the growth and lowers prolactin levels. Asherman syndrome is treated by surgically removing the scar tissue, often during the same procedure used to diagnose it.
Premature ovarian insufficiency is the most complex situation. Because the ovaries have lost most of their function, hormone therapy is generally recommended to replace estrogen and progesterone until the typical age of menopause. This protects bones, cardiovascular health, and overall well-being. Spontaneous pregnancy remains possible in some cases, since ovarian function can occasionally resume unpredictably, but it’s not reliable.