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The reproductive cycle of women is incredibly complex and detailed, and takes around 28-35 days. Many factors play into not only giving birth and pregnancy, but also the menstrual cycle and ovarian cycle, which happen when a woman is not pregnant. Match the pink numbers in the text to the ones on the diagram!
The ovarian cycle and the menstrual cycle are two cycles that overlap and make up the reproductive cycle. The ovarian cycle is responsible for the growth and release of an egg, or the female reproductive cell. The menstrual cycle prepares the uterus for possible pregnancy by thickening the endometrium. These two cycles are synchronized by hormones.
Ovarian cycle: driven by FSH and LH
Pre-ovulatory phase: follicles grow
The ovarian cycle is in charge of preparing an egg for possible pregnancy. It begins when FSH levels begin to rise. The increased levels of FSH are like the alarm clock for follicles to grow. As FSH keeps the follicles growing(1), the follicles begin to secrete low levels of estrogen. As the follicle gets bigger, it gradually secretes more and more estrogen. Throughout this process, FSH and LH levels are fairly low.
Ovulation: rupture of the follicle
Eventually, a single follicle outgrows the rest and begins to secrete much more estrogen(2). When estrogen greatly increases like this, it creates a huge spike in FSH and LH! Up until this point, LH hasn’t done much in the ovarian cycle yet. However, it plays a crucial role when the spike of FSH and LH occurs. The spike in LH causes the follicle to rupture! The rupturing of the follicle is called ovulation(3).
LH: Luteinizing hormone. This hormone signals ovulation, when your body releases the egg from the follicle.
FSH: Follicle Stimulating Hormone. This hormone signals the ovaries to prepare follicles and trigger estrogen production.
Ovulation: The part of the menstrual cycle where the mature egg is released from the ovary.
Follicles: Tiny fluid-filled sacs that protect and nurture eggs to maturity in the ovaries until ovulation. These become the corpus luteum after ovulation and the egg has been released.
Post-ovulatory phase: the follicle becomes the corpus luteum
The ruptured follicle splits into two parts. The first part is an egg that travels down the Fallopian tube for possible pregnancy. The rest of the follicle becomes the corpus luteum, but what’s that? The corpus luteum is a yellow structure that secretes hormones and supports pregnancy in its early stages(it also looks like a pair of puckered lips). Promoted by LH, the corpus luteum secretes estrogen and progesterone(4).
When estrogen and progesterone are both present, FSH and LH levels begin to fall. The fall of FSH and LH accomplishes two things that help to prepare for the next cycle. The first thing is that the decreased levels of FSH and LH stop more follicles from developing before they’re supposed to, and from starting a new cycle. The decreased LH also allows the corpus luteum to slowly degenerate. But why does the corpus luteum need to degenerate? The purpose of a corpus luteum is to support pregnancy, but if pregnancy doesn’t happen, the corpus luteum isn’t needed anymore. This degeneration also decreases the progesterone levels, allowing for menstruation to occur and restarting the cycle again!
Menstrual Cycle: Driven by estrogen and progesterone.
The menstrual cycle overlaps with the ovarian cycle. After the corpus luteum degenerates, estrogen and progesterone levels decrease, which tells the endometrium to shed and for menstruation to begin(5). Corresponding with the start of the pre-ovulatory phase(see above) and the beginning of the growth of the follicles, the menstrual cycle begins with the breakdown of the endometrium in the first 3-5 days of the reproductive cycle. You might recognize this as the monthly uterine bleeding we call periods! During this time, your body discharges blood, small clusters of endometrial cells, and mucus. After the endometrium has broken down, it needs to be regenerated, preparing for a possible pregnancy in the next cycle. This thickening is signaled by rising levels of estrogen and progesterone secreted by the follicles and the corpus luteum throughout the menstrual cycle.
The reproductive cycle of the female human body is amazing and incredibly complex. There’s much more that goes into it than what we covered here today, and we highly recommend you explore more into the mechanisms of the menstrual cycle!
Difficult words:
Corpus luteum: Pronounced: “KOR-puhs LOO-tee-umm.” Definition: The temporary organ that forms from a ruptured follicle after ovulation and produces hormones. It supports early pregnancy and degenerates if pregnancy does not occur.
Egg: Definition: Also known as the ovum, the female reproductive cell/gamete that is fertilized by the sperm and implants on the endometrium in the uterus to start pregnancy.
Endometrium: Pronounced: “en-dow-MEE-tree-umm.” Definition: The inner lining of the uterus that is regrown and then shed during the menstrual cycle to prepare for possible pregnancy.
Estrogen: Pronounced: “EH-struh-jin.” Definition: The sex hormone that develops and regulates the female reproductive system.
Fallopian Tube: Pronounced: “fuh-LOW-pee-uhn TOOB.” Definition: The hollow tubes that connect the ovaries to the uterus, where the egg travels to the uterus, and where fertilization typically takes place.
Female Reproductive Cycle: Definition: The cycle lasting around 28 days that prepares the body for possible pregnancy. Driven by hormones.
Follicles: Pronounced: “FAA-luh-klz.” Definition: Tiny fluid-filled sacs that protect and nurture eggs to maturity in the ovaries until ovulation. These become the corpus luteum after ovulation, and the egg has been released.
FSH: Stands for “follicle-stimulating hormone.” Definition: The hormone that signals the ovaries to prepare follicles and triggers estrogen production.Hormone(s): Pronounced: “HOR-mown.” Definition: the chemical messengers that regulate biological processes like reproduction, growth, and metabolism.
LH: Stands for “luteinizing hormone.” Definition: The hormone that signals ovulation.
Menstrual cycle: Definition: The cycle that prepares the uterus for possible pregnancy and takes about 28 days. It is driven by hormones and involves menstruation and the thickening of the endometrium.
Menstruation: Pronounced: “Men-stur-a-tion.” Definition: The process where a woman discharges blood and other materials lining the uterus. This happens about once a month from puberty until menopause. Menstruation stops during pregnancy.
Ovarian cycle: Pronounced: “oh-VAIR-ee-uhn SY-kuhl.” Definition: The cycle that prepares an egg for possible pregnancy during the female reproductive cycle. During this cycle, the ovaries mature, and follicles grow before releasing a single egg during ovulation.
Ovaries: Pronounced: “OW-vr-eez,” Definition: The pair of small organs in the female reproductive system that produce eggs and hormones.
Ovulation: Pronounced: “aav-you-LAY-shn.” Definition: The part of the menstrual cycle where the mature egg is released from the ovary. Signaled by the spike in FSH and LH.
Pregnancy: Definition: The development of a baby inside the mother’s womb that lasts around 40 weeks.
Progesterone: Pronounced: “pro-JEH-str-own.” Definition: A hormone that regulates the menstrual cycle and supports pregnancy.
Uterus: Pronounced: “YOU-ter-us.” Definition: The pear-shaped hollow organ inside a woman’s pelvis. This is where the baby grows, also called the womb.
Sources:
Taylor, Martha R., et al. Campbell Biology: Concepts & Connections. 10th ed., Pearson, 2021.
https://www.ncbi.nlm.nih.gov/books/NBK279054/
https://www.ncbi.nlm.nih.gov/books/NBK500020/
https://my.clevelandclinic.org/health/articles/24638-follicle-stimulating-hormone-fsh