First period
When the first period arrives
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ACOG describes the median age at menarche as between 12 and 13 years in well-nourished populations, with a median of 12.43 years. By age 15, 98% have started, and that is within three years of thelarche for most.
Timing varies with genetics, nutrition, body mass index, chronic illness, athletic training load, and socioeconomic conditions. Higher gain in body mass index during childhood is associated with earlier onset. Timing also differs by ancestry within the same population, so a single number describes a population and not an individual.
The first period is not the start of fertility, and it is also not proof against it. Menarche marks the point at which the endometrium first responds visibly — not the point at which ovulation begins. The two are decoupled in both directions. Many early cycles are anovulatory, so conception is less likely in the first year or two; but anovulation is not reliable, and ovulation can precede the first bleed entirely, which means pregnancy is possible in someone who has never menstruated. Neither reassurance nor alarm is warranted. The accurate statement is that fertility at this stage is variable and unpredictable.