The reproductive phase of life in humans is different for males and females. In humans, males and females become sexually active when they reach adolescence. At this stage of life, they become capable of reproduction. For males the testes and for females, the ovaries begin to produce gametes. The capacity for maturation and production of gametes lasts for much longer in males over females. In males, production of sperms start at the age of between the age of 12-16 and continue throughout the entire lifetime. The phases of female reproductive cycle start around 10-12 years of age and last till 45-50 years. The ova or the egg begins to mature with the onset of puberty. One ova matures and is released by the ovary every 28-30 days.

During this period, the wall of the uterus becomes thick so it can receive the egg (in case the egg is fertilised and begins to develop). A fertilised egg results in pregnancy and if it is not fertilised then the released egg and the thickened lining of the uterus along with the blood vessels are shed off. This takes place through a process known as menstruation and it generally occurs every 28-30 days. The first menstrual flow that begins at the start of puberty is known as menarche and it continues till 45-50 years (give or take). The stoppage of menstruation is known as menopause. At first, the menstrual cycle can be irregular, however, in time, it becomes regular.

Women's Reproductive Life Cycle- Menstrual Cycle Phases

The day count for the menstrual cycle in adolescent females begins on the first day of menstruation when blood comes out of the vagina. The entire length of the cycle is around 28 days (taken as an average in all females). The entire duration of one menstrual cycle is divided into 4 phases. These are the following:

Menstrual phase (Day 1 to 5)
Follicular phase (Day 1 to 13)
Ovulation phase (Day 14)
Luteal phase (Day 15 to 28)

Menstrual Phase

The menstrual phase starts on the first day of the menstruation and continues until the 5th day of the menstrual cycle.
It starts as the uterus shed the inner soft tissue and blood vessels which exits the body from the vagina as menstrual fluid.
Around 10 ml to 80ml of blood loss takes place.
Adolescent females may experience abdominal cramps during this phase which are caused by the contraction of the uterine and the abdominal muscles to expel the menstrual fluid.

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Follicular Phase

The start of the follicular phase coincides with the first day of the menstruation. However, the phase lasts until the 13th day of the complete menstrual cycle.
In this phase, the pituitary gland secretes a hormone that stimulates egg cells in the ovaries to grow.
One of the egg cells starts to mature in a sac-like structure called the follicle and it takes full 13 days for the egg cell to reach complete maturation.
When the egg cell matures the follicle secretes a hormone that stimulates the uterus to develop a lining of blood vessels and the endometrium (soft tissue)

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Ovulation Phase

This stage takes place on the 14th day of the cycle.
The phase starts as the pituitary gland secretes a hormone that causes the ovary to release the mature egg cell which is then swept into the fallopian tube. This takes place with the help of the cilia of the fimbriae.
Fimbriae are finger-like projections which are observed at the end of the fallopian tube close to the ovaries. The cilia are hair-like projections found on the fimbriae.
Ovulation is triggered by LH.

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Luteal Phase

The luteal phase begins on the 15 the day of the menstrual cycle.
The mature egg cell is released during the ovulation and the follicle is ruptured and it stays on the surface of the ovary.
In the next two week and the follicle transforms into a structure known as corpus luteum.
The corpus luteum at this time releases progesterone with small amounts of oestrogen. The hormones maintain the thickened line of the uterus whilst waiting for a fertilised egg to implant on the wall.
If a fertilised egg is implanted on the wall of the uterus, it starts producing hormones which are necessary to maintain corpus luteum. (one such hormone is human chorionic gonadotropin)
If pregnancy does not occur then the egg cell disintegrates along with the corpus luteum. (Corpus luteum withers and dies). There’s a drop in progesterone levels which causes the lining of the uterus to fall away in a process called menstruation and then the cycle starts again.

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FAQs on Menstrual Cycle Reproductive Phase Explained

1. What is the menstrual cycle and what is its primary function in the reproductive phase?

The menstrual cycle is a series of natural changes in the female reproductive system that occurs approximately every 28 days. Its primary function is to prepare the body for a potential pregnancy. This involves the maturation and release of an egg (ovum) from the ovaries and the preparation of the uterine lining (endometrium) to receive a fertilised egg. If pregnancy does not occur, the cycle concludes with the shedding of the uterine lining, known as menstruation.

2. What are the four main phases of the menstrual cycle?

The menstrual cycle is divided into four distinct phases, each regulated by specific hormones:

Menstrual Phase (Days 1-5): This phase begins on the first day of the period. The uterine lining (endometrium) is shed, resulting in menstrual bleeding.
Follicular Phase (Days 1-13): Overlapping with the menstrual phase, this is when follicles in the ovary mature. One dominant follicle prepares to release an egg. The hormone FSH (Follicle-Stimulating Hormone) is key here.
Ovulation Phase (Day 14): A surge in LH (Luteinizing Hormone) triggers the release of a mature egg from the ovary into the fallopian tube. This is the most fertile time in the cycle.
Luteal Phase (Days 15-28): After ovulation, the empty follicle develops into the corpus luteum, which produces progesterone. This hormone thickens the uterine lining to prepare for implantation.

3. What is the difference between the ovarian cycle and the uterine (endometrial) cycle?

The menstrual cycle actually consists of two synchronised cycles happening simultaneously:

The ovarian cycle describes the events in the ovaries, specifically the maturation and release of an egg. Its phases are the follicular phase, ovulation, and luteal phase.
The uterine cycle describes the changes in the uterus lining (endometrium). Its phases are the menstrual phase (shedding), proliferative phase (rebuilding), and secretory phase (preparation for implantation). These uterine changes are directly controlled by hormones produced during the ovarian cycle.

4. How do hormones like LH, FSH, estrogen, and progesterone regulate the menstrual cycle?

These four hormones work in a coordinated feedback loop to control the cycle:

FSH (Follicle-Stimulating Hormone): Secreted by the pituitary gland, it stimulates the growth of ovarian follicles at the beginning of the cycle.
Estrogen: Produced by the growing follicles, it causes the uterine lining to thicken (proliferate) and, at high levels, triggers the LH surge.
LH (Luteinizing Hormone): A sharp surge of this pituitary hormone triggers ovulation, causing the mature follicle to rupture and release the egg.
Progesterone: Produced by the corpus luteum after ovulation, it maintains the thickened uterine lining, making it receptive to a fertilised egg. If pregnancy doesn't occur, progesterone levels fall, triggering menstruation.

5. What is the importance of the luteal phase, and what happens if fertilisation does not occur?

The luteal phase is crucial for establishing a potential pregnancy. Its primary importance is the production of progesterone by the corpus luteum. This hormone makes the uterine lining (endometrium) thick, nutrient-rich, and stable, creating an ideal environment for a fertilised egg to implant and develop. If fertilisation does not occur, the corpus luteum degenerates after about 10-12 days. This leads to a sharp drop in progesterone and estrogen levels, which causes the uterine lining to break down and shed, initiating the menstrual phase and starting a new cycle.

6. Why is a regular menstrual cycle considered an important indicator of reproductive health?

A regular menstrual cycle is a key indicator of reproductive health because it reflects a well-functioning and balanced hypothalamic-pituitary-ovarian (HPO) axis. Regularity suggests that the body is producing the necessary hormones (FSH, LH, estrogen, and progesterone) in the correct sequence and amounts to facilitate follicle development and ovulation. Conversely, irregularities like very long, short, or absent cycles can be the first sign of underlying health issues, hormonal imbalances, or conditions such as Polycystic Ovary Syndrome (PCOS).

7. What are some common conditions associated with the menstrual cycle?

Several common conditions are related to the menstrual cycle's hormonal fluctuations:

Premenstrual Syndrome (PMS): A group of symptoms, including mood swings, fatigue, and bloating, that occur in the days leading up to menstruation due to hormonal changes.
Dysmenorrhoea: Refers to painful periods, often caused by strong uterine contractions triggered by substances called prostaglandins.
Amenorrhoea: The absence of menstrual periods. It is normal before puberty, during pregnancy, and after menopause, but can indicate a health issue at other times, often related to low body weight, excessive exercise, or stress.

8. Can external factors like stress or diet impact the menstrual cycle?

Yes, external factors can significantly impact the menstrual cycle. The hypothalamus, a region in the brain that helps regulate the cycle, is sensitive to factors like emotional stress, extreme diet changes, and excessive exercise. High levels of stress can disrupt the hormonal signals from the brain to the pituitary gland, potentially delaying or even preventing ovulation. Similarly, poor nutrition or very low body fat can interfere with hormone production, leading to irregular or absent periods (amenorrhoea).