Question

What is a menstrual cycle? When does it begin? What happens during this period?

Answer 1

Hint: Menstrual cycle is the regular natural change that occurs in the female reproductive system (both uterus and ovaries) which makes pregnancy possible. Girls start to have a menstrual cycle when puberty begins.

Complete answer:
Menstrual cycle is a series of changes that the uterine endothelium goes through every month in response to the uterine hormones. The cycle depends upon the rise and the fall of estrogen levels. It is required for the production of oocytes and the preparation of the uterus for pregnancy.

A menstrual cycle starts with the first day of the menstrual phase (menstrual flow) and ends with the start of the next menstrual phase. A menstrual cycle usually lasts between 24 to 38 days but the length may vary from cycle to cycle over the years.

The different phases of menstrual cycle include:
> Menstrual phase (Days 1-5): The thick stratum functionalis detaches. The detached tissue and blood pass out through the vagina as the menstrual flow for 3-5 days. By the fifth day, the growing ovarian follicles produce more estrogen. Gonadotropins and ovarian hormones are at low levels.

> Proliferative phase (Days 6-14): The stratum basalis of the endometrium generates a new stratum functionalis under the influence of rising blood levels of estrogen. As the new layer thickens, blood vessels enter the stratum functionalis and the gland enlarges. Ovulation has happened. Cervical mucous undergoes thinning due to estrogen. Progesterone receptors on the endometrium are ready to detect and respond to the secreted progesterone.

> Secretory phase (Day 15-28): The uterine gland enlarges (hypertrophy) and secretes uterine milk for nourishment of the embryo. The spiral blood vessels in the stratum functionalis are well developed and ready for placenta formation.

Note: In case of no fertilization, the stratum functionalis gets destroyed. The blood vessels and glands break and get discharged as menstrual fluid. This marks the end of one menstrual cycle.