Disclaimer

All references to female, girl, woman, etc. are in reference to individuals of the female sex, or in possession of two X chromosomes.

TL;DR

Females are born with all the eggs they will ever have. Once a girl reaches puberty, a single dominant egg completes development, while the uterus creates a vascular environment for a freshly fertilized zygote. The mature egg bursts from its follicle like a baby bird leaving its nest, gets caught by the fimbriae that bring the egg to the fallopian tube, to be potentially fertilized on its journey to the uterus. If fertilized, the zygote attaches to the wall of the uterus where a placenta begins development, and rapid mitosis occurs to form a fetus. If left unfertilized, the egg breaks down, and the uterus sheds its lining.

The “Normal” Female Reproductive Cycle

Ovaries are typically the size of a Greek olive – within them are all the eggs, or oocytes. During fetal development, 1-2 million stem cells develop into primary oocytes and undergo meiosis I, arresting at the prophase I stage until a girl reaches puberty. Females are born with the most eggs they will ever have, frozen in prophase I until ovulation/menarche. The female reproductive cycle is split into two phases, the follicular phase, and the luteal phase. Once a female reaches puberty, the ovaries release a group of eggs from their “vault.”

Graph depicting hormone levels during a 28 day cycle; Figure depicting follicle development through ovulation the lifecycle of the corpus luteum

Follicular Phase (~Days 1-14)

What:

The start of the follicular phase is the first day of full menstrual bleeding. The phase received this name as it is the phase where the follicle, or the fluid filled structures containing a single egg is stimulated by the appropriately named, Follicle Stimulating Hormone (FSH) to grow. Multiple follicles are recruited to grow, but by days 5-7, a single dominant follicle is selected and the remaining cohort degenerates, allowing the dominant follicle access to the necessary resources required to complete development. Meanwhile, as the dominant follicle increases in size, the cells surrounding the egg, granulosa cells produce estradiol (estrogen) in response to FSH, which stimulates the functional layer in the uterus to thicken and vascularize, creating a happy home for a developing zygote.

Once the egg is mature enough, the follicle bursts, releasing the egg to be caught by the finger-like tendrils of the fimbria, the baseball mitt of the fallopian tubes. The egg travels down the fallopian tube where it has a 24-hour period to be fertilized by a single sperm.

How:

The hypothalamus releases Gonadotropin Releasing Hormone (GnRH) which stimulates the anterior pituitary gland to release Follicle Stimulating Hormone (FSH). FSH stimulates a single oocyte to complete meiosis I, creating a secondary oocyte. The cells surrounding the developing oocyte produces estrogen. This estrogen acts as a signal to the pituitary gland, and the estrogen levels reach at least 200 pg/mL for 50 hours, the pituitary gland receives this signal, and sends out a surge of luteinizing hormone, stimulating the follicle to burst, releasing the egg, now arrested in metaphase II until fertilization. The LH “surge” leads to ovulation where the mature egg, arrested at metaphase II, gets picked up by the fimbria of the fallopian tubes and travels through the fallopian tubes, ultimately ending up in the uterus.

Luteal Phase (~Days 14-28)

The follicle that grew the oocyte to maturity, forms a cyst called the corpus luteum (hence luteal phase) – and starts to produce progesterone and estrogen. The lifespan of the corpus luteum is about 14 days long. If the egg is fertilized, and attaches to the uterine lining, the zygote produces human chorionic gonadotropin (HcG), extending the lifetime of the corpus luteum until the developing zygote reaches 12 weeks of development, when the placenta takes over progesterone and estrogen production. If the egg remains unfertilized, the corpus luteum degenerates and forms the corpus albicans, essentially, scar tissue, the unfertilized egg degenerates, and the uterine lining is shed over 4-7 days during menses. After menses, the cycle restarts with the recruitment of another cohort of immature primary oocytes housed within follicles.

Don’t like to read?

Dr. Natalie Crawford has an amazing podcast called “As a Woman” – this episode linked below specifically dives into the menstrual cycle.

As a Woman Podcast - Episode 007: The Menstrual Cycle, Period.