The first day of the monthly periods is the first day of menstruation, where
female steroid hormones like estrogen and progesterone are low.
A normal monthly period for adolescents is characterized by time of 21-38 days
between menstrual bleeding and a period of menstrual bleeding lasting 2-7
days, with an expected blood loss of around 20-80 ml.
First menses typically occurs within 2 to 3 years after breast development.
In girls with early start of breast development, the interval to first menses is
longer (3 years or more) than in girls with later start. It is found that by 15 years
of age, 98% of females will have had first menses.
A number of medical conditions can cause irregular or missed menses in some young females.
Irregular menses may be linked with many conditions, including pregnancy,
hormone disorders and acquired medical conditions, because all of these conditions are related with improper secretion of female sex hormones.
Commonly, Polycystic Ovary Syndrome (PCOS) causes a long time intervals
between menstrual periods, especially in patients with signs of hormone
disorder in women of reproductive age.