Menstrual Cycle – Class 12 | Chapter – 3 | Biology Short Notes Series PDF

Menstrual Cycle: The menstrual cycle is complex and is controlled by many different glands and the hormones that these glands produce. A brain structure called the hypothalamus causes the nearby pituitary gland to produce certain chemicals, which prompt the ovaries to produce the sex hormones oestrogen and progesterone. The average length of the menstrual cycle is 28–29 days, but this can vary between women and from one cycle to the next. The length of your menstrual cycle is calculated from the first day of your period to the day before your next period starts.


Menstrual Cycle Diagram

How does the menstrual cycle work?

Females of reproductive age (beginning anywhere from 11 to 16 years of age) experience cycles of hormonal activity that repeat at about one-month intervals. The average menstrual cycle takes about 28 days and occurs in phases. These phases include:

  • Menstrual phase (first stage of the menstrual cycle): Day 1, uterus lining which is prepared for implantation starts to shed which lasts 3 to 5 days.
  • Follicular phase (development of the egg):  In this phase, the primary follicle starts developing into a mature Graffian follicle. The endometrium also starts proliferating. The uterus starts preparation for another pregnancy.
  • Ovulatory phase (release of the egg): Mid-cycle phase, this is the phase in which ovulation takes place i.e., day 13-17. The end of the follicular phase along with the ovulation period defines the fertilisation period.
  • Luteal phase (hormone levels decrease if the egg does not implant): It is the post-ovulation phase, where the fate of the corpus luteum is decided. If fertilisation occurs, pregnancy starts. If fertilisation doesn’t occur, it marks the onset of another cycle.

What causes menstrual cycle irregularities?

  • Pregnancy or breast-feeding. A missed period can be an early sign of pregnancy. Breast-feeding typically delays the return of menstruation after pregnancy.
  • Eating disorders, extreme weight loss or excessive exercising. Eating disorders — such as anorexia nervosa — extreme weight loss and increased physical activity can disrupt menstruation.
  • Polycystic ovary syndrome (PCOS). Women with this common endocrine system disorder may have irregular periods as well as enlarged ovaries that contain small collections of fluid — called follicles — located in each ovary as seen during an ultrasound exam.
  • Premature ovarian failure. Premature ovarian failure refers to the loss of normal ovarian function before age 40. Women who have premature ovarian failure — also known as primary ovarian insufficiency — might have irregular or occasional periods for years.
  • Pelvic inflammatory disease (PID). This infection of the reproductive organs can cause irregular menstrual bleeding.
  • Uterine fibroids. Uterine fibroids are noncancerous growths of the uterus. They can cause heavy menstrual periods and prolonged menstrual periods.

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By Team Learning Mantras