The Female Menstrual Cycle

 

The female menstrual cycle is a complex and natural process that plays a central role in reproductive health. It involves a series of hormonal changes that prepare the body for potential pregnancy, and when pregnancy does not occur, the cycle ends with menstruation. Typically, the menstrual cycle lasts around 28 days, but it can range from 21 to 35 days in healthy individuals. Understanding the different phases of the menstrual cycle, the hormonal fluctuations involved, and how they affect the body is crucial for comprehending fertility and overall female health.

Phases of the Menstrual Cycle

The menstrual cycle is divided into four main phases:

1. Menstrual Phase
2. Follicular Phase
3. Ovulation
4. Luteal Phase

Each phase is driven by the interplay of hormones produced by the hypothalamus, pituitary gland, and ovaries. The Society for Endocrinology has an information page on the Menstrual Cycle, here.

1. Menstrual Phase (Days 1-5)

What Happens:

• The menstrual phase marks the beginning of the cycle and starts on the first day of menstruation (bleeding). It occurs when an egg released in the previous cycle is not fertilised.
• The hormone levels (especially oestrogen and progesterone) are at their lowest during this phase. The drop in progesterone triggers the shedding of the uterine lining (endometrium), which results in menstrual bleeding.
• The blood, mucus, and tissue are expelled from the body through the vagina, typically lasting between 3 to 7 days.

Physical Symptoms:

• Women may experience cramps due to the contraction of the uterus as it sheds its lining. These cramps can be mild or severe and are often felt in the lower abdomen, back, or thighs.
• Other symptoms may include bloating, breast tenderness, fatigue, and mood swings due to hormonal fluctuations.

2. Follicular Phase (Days 1-13)

What Happens:

• The follicular phase overlaps with the menstrual phase in the early part of the cycle. It begins on the first day of menstruation and continues until ovulation (about day 14 in a 28-day cycle).
• This phase is named after the "follicles" in the ovaries, which house immature eggs. During the follicular phase, the pituitary gland in the brain releases follicle-stimulating hormone (FSH), which stimulates the growth of several follicles in the ovaries.
• Out of the several follicles that start growing, typically only one becomes dominant and continues to mature. This follicle will eventually release an egg during ovulation.
• As the follicle grows, it produces oestrogen. Rising oestrogen levels cause the endometrium (uterine lining) to thicken in preparation for a potential pregnancy.

Hormonal Activity:

• FSH stimulates the growth of ovarian follicles.
• Oestrogen rises gradually, leading to the thickening of the uterine lining.

Physical and Emotional Symptoms:

• This phase is generally associated with increased energy and improved mood as oestrogen levels rise. Some women may notice improved skin and hair texture, as well as a stronger libido.
• Vaginal discharge may increase and become clearer and more slippery as ovulation approaches, which helps sperm travel through the reproductive tract.

3. Ovulation (Day 14)

What Happens:

• Ovulation is the midpoint of the cycle and occurs when the mature follicle bursts, releasing an egg from one of the ovaries. This egg travels through the fallopian tube, where it may encounter sperm and become fertilised.
• Ovulation is triggered by a surge in luteinizing hormone (LH), which is produced by the pituitary gland. The LH surge typically occurs about 24 to 36 hours before ovulation.
• Once released, the egg has about 12 to 24 hours to be fertilised. If sperm is present in the fallopian tube during this time, fertilization may occur.

Hormonal Activity:

• LH surge triggers the release of the mature egg from the ovary.
• Oestrogen levels peak just before ovulation, and then begin to decline after the release of the egg.

Physical Symptoms:

• Many women experience a slight increase in basal body temperature after ovulation due to the rise in progesterone.
• Some women report mild abdominal pain or discomfort on one side of the pelvis, known as "mittelschmerz," as the egg is released.
• There may be an increase in vaginal discharge, which is often clear, stretchy, and resembles egg whites, making it easier for sperm to travel through the cervix.

Fertility Window:

• The days surrounding ovulation are considered the most fertile period of the menstrual cycle. Sperm can survive inside the female reproductive system for up to 5 days, so the "fertility window" includes the 5 days before ovulation and the day of ovulation itself.

4. Luteal Phase (Days 15-28)

What Happens:

• After ovulation, the ruptured follicle transforms into the corpus luteum, which produces progesterone. Progesterone is essential for maintaining the thickened uterine lining and preparing it for the implantation of a fertilised egg.
• If the egg is fertilised, it will implant into the uterine lining about 6-12 days after ovulation. The body will then start producing human chorionic gonadotropin (hCG), the hormone that supports pregnancy.
• If the egg is not fertilised, the corpus luteum breaks down, and progesterone levels begin to fall. This drop in progesterone causes the uterine lining to shed, leading to menstruation and the start of a new cycle.

Hormonal Activity:

• Progesterone dominates the luteal phase, maintaining the uterine lining in case of pregnancy.
• Oestrogen levels also rise slightly but are lower than in the follicular phase.

Physical Symptoms:

• The luteal phase is often associated with premenstrual symptoms (PMS), including mood swings, irritability, bloating, headaches, breast tenderness, and fatigue.
• Some women may also experience food cravings or changes in appetite. These symptoms are largely due to the decline in progesterone and oestrogen as the body prepares for menstruation if pregnancy does not occur.

Hormones Involved in the Menstrual Cycle

1. Gonadotropin-Releasing Hormone (GnRH): Produced by the hypothalamus, GnRH stimulates the pituitary gland to release FSH and LH, which are essential for follicle development and ovulation.
2. Follicle-Stimulating Hormone (FSH): This hormone stimulates the growth of ovarian follicles in the follicular phase.
3. Luteinising Hormone (LH): The surge in LH triggers ovulation and the release of the egg from the mature follicle.
4. Oestrogen: Primarily produced by the growing follicle, oestrogen helps thicken the uterine lining and plays a role in regulating the release of LH.
5. Progesterone: Produced by the corpus luteum in the luteal phase, progesterone maintains the uterine lining and prepares it for the possible implantation of a fertilised egg.
6. Prostaglandins: These are lipid compounds that cause the uterus to contract during menstruation, leading to the shedding of the uterine lining and often contributing to menstrual cramps.

The Cleveland Clinic, has more information on menstruation, here. They also have some excellent information on what can go wrong in the cycle, here.

Conclusion

The female menstrual cycle is a complex interplay of hormones that regulate ovulation, prepare the body for potential pregnancy, and, in the absence of pregnancy, initiate menstruation. Each phase of the cycle serves a specific purpose in ensuring reproductive health, with precise hormonal changes influencing both physical and emotional symptoms. Understanding the menstrual cycle can help women track fertility, manage symptoms, and recognise any irregularities that might indicate underlying health issues. While the average cycle length is 28 days, variations are normal, and cycle length can change due to factors like stress, illness, and lifestyle. The University of California San Francisco has a section their website offering additional information, here.