Menarche refers to the first menstrual period, or first menstrual bleeding, as a female's body progresses through the changes of puberty. Menarche usually occurs about two years after the first changes of breast development (thelarche).

When menarche occurs, it confirms that the female has had a gradual estrogen-induced growth of the uterus, especially the endometrium, and that the "outflow tract" from the uterus, through the cervix to the vagina, is open.

Menarche and subsequent menstrual periods (menses) may occur without ovulation, but there have been rare instances of ovulation preceding menarche by 1–2 weeks.

Menarche is the most commonly remembered milestone of puberty for most women. The average age of menarche in the United States is about 12 years, 8 months, which is a few months earlier than fifty years ago.

In very rare instances, menarche may occur at an unusually early age, preceding thelarche and other signs of puberty. This is termed isolated premature menarche, but other causes of bleeding must be investigated and excluded.

When menarche has failed to occur for more than 3 years after thelarche, or past 14.5 years of age, the delay is referred to as primary amenorrhea.

See also puberty, delayed puberty, precocious puberty, thelarche, adrenarche, pubarche, gonadarche, menstrual cycle.