The cycle of an 18-year-old woman is still unstable.
The first menstruation is an important sign of puberty, usually when a woman is close to 10 years old when her breasts start to develop, the first menstruation is on average 2.5 years later than breast development, because at this time the center of the positive feedback mechanism of estrogen has not yet matured, so the menstrual cycle is often irregular, and after 5 to 7 years of the establishment of regular cyclic ovulation, menstruation is only gradually normal.
At 18 years of age, the ovaries and uterus may not be fully developed, estrogen and progesterone may still be in an unstable state, and androgen levels are slightly higher, so they may not have established a stable and regular menstrual cycle, but generally speaking, most of the girls at 18 years of age have already established a relatively stable menstrual cycle.
The normal menstrual cycle is 21 to 35 days, in a fluctuating state, and menstruation can be affected by many factors, such as stress, environment, emotions, staying up late, diet, etc. However, if you have not established a stable menstrual cycle at the age of 20, you may not have a stable and regular menstrual cycle.
However, if you have not established a stable cycle by the age of 20, it is important to seek medical attention to rule out disease-based menstrual irregularities such as polycystic ovary syndrome, which can also lead to irregular menstruation or even amenorrhea.