Understanding the Menstrual Cycle

The female reproductive system undergoes dynamic changes on a 25-32 monthly rhythm, which we know as the menstrual cycle. This cycle plays a critical role in fertility and overall reproductive health and is characterized by two key phases, the follicular and luteal. These phases help to prepare the body for potential pregnancy and can influence various aspects of well-being. 

Understanding these changes is helpful when considering fertility, PMS, and other hormone-related health concerns.

Phases of the Menstrual Cycle & Their Evolutionary Role in Fertility

The menstrual cycle consists of four distinct phases, each regulated by specific hormonal patterns:

Follicular Phase (Days 1-14, overlapping with menstruation)

The follicular phase starts with the first day of menstruation, where the uterine lining (endometrium) sheds and menstrual bleeding results. Menstruation typically lasts 3-5 days and marks the beginning of a new cycle after a drop in estrogen and progesterone levels indicate that pregnancy has not occurred. Evolutionarily, this phase resets the reproductive system, preparing it for a potential new pregnancy in the following cycle.

After menstruation is complete, the pituitary gland begins to releases follicle-stimulating hormone (FSH) which prompts the ovaries to produce and mature several follicles. Of the many follicles that are stimulated, one dominant follicle will continue to develop and prepare for ovulation. As the follicular phase progresses, estrogen levels rise and the endometrial lining thickens in anticipation of potential implantation. 

Ovulation occurs around day 14 of the cycle when a surge in luteinizing hormone (LH) triggers the release of a mature egg from the dominant follicle. Estrogen peaks just before ovulation, and a slight increase in testosterone enhances libido and energy levels, promoting reproductive success. This is the fertile window, when conception is most likely to occur, emphasizing its critical role in evolutionary biology.

Luteal Phase (Days 15-28)

After ovulation, the ruptured follicle shrinks and transforms into what is called the corpus luteum. The role of the corpus luteum is to secrete progesterone which remains the dominant hormone during this phase, while estrogen levels are moderate. If fertilization does not occur, progesterone levels drop, signaling the body to shed the uterine lining and initiate a new menstrual cycle.

The goal of this phase is to prepare the body for pregnancy and allow for an efficient reset if conception does not take place.

Understanding the Impact of Hormone Imbalances

Disruptions in hormone signaling due to stress, malfunction of an endocrine gland (ie. Thyroid, ovaries, pancreas, etc.), or exposure to endocrine disrupting chemicals from the environment can contribute to various reproductive and metabolic concerns, including ovulatory dysfunction, PMS, and estrogen dominance.

1. Ovulatory Dysfunction

Irregular ovulation (aka: anovulation) can result from conditions such as polycystic ovary syndrome (PCOS), thyroid imbalances, excessive stress, or low body fat. These changes can lead to irregular cycles, difficulty conceiving, and imbalances in the production of estrogen and progesterone.

2. Premenstrual Syndrome (PMS) & PMDD

PMS and its more severe form, premenstrual dysphoric disorder (PMDD), arise from fluctuations in estrogen, progesterone, and neurotransmitters such as serotonin. Symptoms may include mood swings, bloating, headaches, and heightened emotional sensitivity, affecting daily life and well-being.

3. Estrogen Dominance

Estrogen dominance occurs when the body senses estrogen unopposed by progesterone. Although it is more common for estrogen levels to be elevated, estrogen dominance symptoms can also occur if luteal phase progesterone levels are low. This imbalance can result from poor estrogen metabolism, high stress levels, or environmental toxin exposure. Symptoms often include heavy periods, fibroids, breast tenderness, and increased PMS severity.

4. Luteal Phase Deficiency

A shortened luteal phase or insufficient progesterone production can impair implantation and early pregnancy maintenance. Chronic stress and nutrient deficiencies are common contributors, often leading to spotting before menstruation and challenges with fertility.

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References:

• Thiyagarajan DK, Basit H, Jeanmonod R. Physiology, menstrual cycle. In: StatPearls. StatPearls Publishing; 2025.

• Welt, CK. Barbieri, RL. Martin KA. Normal Menstrual Cycle. UpToDate. Current through: Feb 2025. Updated: April 25th, 2024.