Most girls get their first period around age 12, but the normal range is wide. About 10% of girls start by age 10, while 10% haven’t started by age 14. If your daughter falls on the later end of that spectrum, it’s likely normal, especially if she’s showing other signs of puberty. The timing depends on genetics, body composition, nutrition, and activity level.
What’s Considered Normal Timing
The median age of a first period in the United States is 11 years and 10 months. By age 12, roughly half of girls have started menstruating. By age 14, about 90% have. That leaves a full 10% of healthy girls who simply haven’t gotten there yet at 14, and many of them will start within the next year or so without any intervention.
The most reliable predictor isn’t age alone. It’s where your daughter is in puberty overall. The first period typically arrives about 2 to 2.5 years after breast development begins, with a range of 6 months to 3 years. If your daughter started developing breast buds at 11, a first period at 13 or 14 is right on schedule. If she hasn’t shown any signs of puberty at all (no breast development, no pubic or underarm hair) by age 13, that’s the point where a medical evaluation is worthwhile.
When Late Becomes a Medical Concern
Doctors use two specific thresholds to decide when a late period warrants investigation. The first: if your daughter has visible signs of puberty (breast development, body hair) but hasn’t had a period by age 15, or within 3 years of breast development starting. The second: if she has no signs of puberty at all by age 13. Either of these situations is called primary amenorrhea, and it simply means the first period hasn’t arrived within the expected window. It’s a starting point for figuring out why, not a diagnosis in itself.
The Most Common Reasons for a Late Period
Constitutional Delay
Sometimes called being a “late bloomer,” this accounts for roughly 14% of cases when doctors evaluate delayed periods. Everything is developing normally, just on a slower timeline. This often runs in families. If you or your daughter’s biological father went through puberty late, there’s a good chance she will too. In these cases, the period eventually arrives on its own without treatment.
Polycystic Ovary Syndrome (PCOS)
PCOS is actually the single most common medical cause of a delayed first period, accounting for about 32% of cases in one large analysis. In PCOS, the body produces higher levels of androgens (often called “male hormones,” though all women produce some). This can interfere with the hormonal signals that trigger menstruation. Girls with PCOS may have other signs like acne, excess facial or body hair, or weight gain, but not always.
Low Energy Availability
The body needs a certain amount of stored energy to support a menstrual cycle. Research suggests girls need a body fat percentage of roughly 17% for their first period to start, and around 22% to maintain regular cycles afterward. When a girl isn’t eating enough to fuel both her daily activity and her body’s growth needs, the brain dials down reproductive hormones. This is the body’s way of saying “now is not a safe time for reproduction.”
This is especially common in athletes. Among girls evaluated for delayed periods tied to low hormonal signaling, the most frequent cause (38%) was relative energy deficiency in sport, where intense training outpaces calorie intake. Eating disorders accounted for another 17%. But a girl doesn’t need to have a diagnosed eating disorder for this to happen. Picky eating, chronic undereating, or simply not keeping up with the caloric demands of a growth spurt can be enough.
Nutritional status during childhood can explain as much as 25% of the variation in when puberty happens. Chronic undernutrition lowers the hormones that drive puberty forward, particularly a hormone called leptin (produced by fat cells) that signals the brain it’s safe to activate the reproductive system.
Hormonal and Thyroid Issues
The signal chain that triggers a period starts in the brain and travels through the pituitary gland to the ovaries. A disruption at any point along this chain can delay things. Thyroid problems, elevated levels of the hormone prolactin, or problems with the pituitary gland itself can all interfere. These causes are less common (the pituitary accounts for about 3% of cases, other hormonal causes about 6%) but are straightforward to identify with blood tests.
Chromosomal and Anatomical Differences
In a small number of cases, a structural or genetic difference is responsible. Turner syndrome, which affects about 1 in 2,500 girls, involves a missing or incomplete X chromosome and often prevents the ovaries from developing fully. Girls with Turner syndrome are typically shorter than average and may not go through puberty without hormonal support. Anatomical differences in the uterus or reproductive tract are even rarer, accounting for roughly 1.5% of cases.
What a Doctor Will Check
If your daughter’s doctor recommends an evaluation, the first step is usually an assessment of her current stage of puberty, looking at breast development and body hair to gauge how far along she is. A wrist X-ray is commonly used to check “bone age,” which shows whether her skeleton is maturing on schedule, ahead, or behind. Blood tests measuring two key reproductive hormones (FSH and LH) help determine whether the delay originates in the brain’s signaling system or in the ovaries themselves.
Depending on those initial results, additional tests might include thyroid function, a pelvic ultrasound to check the uterus and ovaries, or in some cases a chromosomal analysis. If the doctor suspects a problem with the pituitary gland, an MRI of the brain may be ordered. The process is methodical, working from the most common causes toward the rarer ones.
Why Bone Health Matters Right Now
One reason doctors take a delayed period seriously is its connection to bone development. About 25% of a woman’s lifetime bone mass is built in the two years surrounding her first period. The hormones that drive menstruation, particularly estrogen, are the same ones that strengthen bones during adolescence. A significant delay means your daughter may be missing a critical window for bone growth, which can affect her bone density for the rest of her life. This is especially relevant for young athletes, where the combination of intense training, low body fat, and absent periods can set the stage for stress fractures and long-term bone weakness.
What You Can Do Now
If your daughter is under 14 and showing other signs of puberty, the most likely answer is that her body just isn’t quite there yet. Keep an eye on her overall development and give it time.
If she’s 14 or older with signs of puberty but no period, or 13 with no signs of puberty at all, schedule an appointment with her pediatrician or a pediatric endocrinologist. Make sure she’s eating enough to support her growth, particularly if she’s physically active. Girls going through puberty need significantly more calories than they did as younger children, and restricting food during this window, whether intentionally or accidentally, can meaningfully delay development.
Family history is one of the most useful data points you can bring to a doctor’s visit. Knowing when you, your mother, or your daughter’s biological aunts started their periods gives the doctor context for whether her timeline fits a familial pattern or suggests something else is going on.