Chapter I: How does the menstrual cycle work?

There a four main factors varying throughout the cycle which are all necessary for reproduction. Namely, menstruation, ovulation, cervical mucus and body temperature. The first day of menstruation, when your period starts, is considered as the first day of the cycle and menstruation is the discharge of blood and tissue from the uterus through the vagina that lasts 3-5 days on average. Most popular female hygiene products during this time include pads, tampons or menstrual cups. During the cycle, the endometrium thickens to be prepared for a potential pregnancy and if no pregnancy occurs, a new menstrual cycle begins with the first day of bleeding.

For pregnancy, it is necessary that an egg cell fuses with a sperm followed by forming a zygote that implants in the uterine wall and uterine lining where it resides and its growth and development until birth takes place. Before fertilization, the mature egg cell needs to be released from the ovaries which is called ovulation. Under the right conditions, sperm can survive 2-6 days in the woman’s reproductive system whereas a released egg only survives up to 13-25 hours if not fertilized. From this you can calculate a fertile window, starting approximately 4 days before ovulation and closing 1 day after ovulation. The first part of the cycle, where the egg matures until ovulation, is named the follicular phase.

The second part of the cycle, from ovulation until the next menstruation, is the luteal phase. Contrary to common belief though, ovulation does NOT always take place on day 14 of a woman's menstrual cycle. This estimation is built on statistics and average, but every woman and her cycle are different and the ovulation date can vary each cycle.

Another important parameter that is dynamic during the cycle is cervical mucus. Cervical mucus is secreted by the cervix and exits the body through the vagina. It lubricates and protects the vagina from bacteria and viruses. The amount and consistency of the cervical mucus changes over the cycle to make it easy or difficult for the sperm to enter the uterus and reach the egg for fertilization.

At the beginning of the cycle, right after your period, the amount is little and the consistency is thick and sticky. When you reach ovulation, more mucus is produced and it becomes clearer and more elastic. During this time, the consistency, amount and even pH value of the mucus provide the best conditions for the sperm to move and survive. After ovulation, consistency changes back to thick and sticky and continues to decrease until the end of menstruation.

The last parameter changing during the cycle, is the body temperature. The basal body temperature increases of at least 0.4 °F (that is about 0.22 °C) during ovulation. Some also use BBT charting as a form of birth control and ovulation prediction method, but it is important to know that body temperature is prone to many different factors. You can find more information on BBT here.

All these changes occur during the cycle to enable reproduction. But how are they regulated? The answer is: through hormones and the endocrine system! There are several sexual hormones that regulate reproduction by controlling the uterine and ovarian cycles as a complex system. The hormones that influence your reproductive system are originally regulated by a region in the brain, the hypothalamus.

The hypothalamus synthesizes and secretes hormones such as gonadotropin-releasing hormone (GnRH). GnRH signals to the pituitary to release the hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The pituitary is an endocrine gland which is located underneath the brain and has approximately the size of a pea. LH and FSH in turn, trigger the release of estrogen and progesterone from the ovaries. Like a circle, estrogen and progesterone feedback to the hypothalamus and pituitary to regulate the release of the respective hormones.

This self-regulating system is called the hypothalamic–pituitary–gonadal (HPG) axis. The levels of the hormones rise and drop during different stage of the cycle to trigger ovulation and prepare the body for a potential pregnancy. High levels of FSH in the beginning of the follicular phase stimulate folliculogenesis. Also estrogen levels are high at the beginning of the cycle and aid maturing the follicle to prepare it for ovulation. High levels of estrogen and a sharp peak of LH trigger ovulation approximately 24 hours later.

After ovulation, during the luteal phase, estrogen and LH levels drop. Progesterone levels are high to prepare the uterus’ inner layer for implantation in case the egg got fertilized by a sperm. The HPG axis is very sensitive to environmental factors such as stress, physical exercise, and weight gain or weight loss. Deregulation of the HPG axis can lead to impaired ovarian and uterine function. The so called HPG axis can also be suppressed artificially by the hormonal birth control administration. Naturally, the hormones also change when implantation and pregnancy occurs.

Be sure to keep a eye out for Chapter II of this series: How does the pill work ?