Ovaries: small, almond-shaped structures found at the end of the Fallopian tubes that produce/house the eggs and secrete hormones.
Fallopian tubes: tubes that connect the ovaries with the uterus. This is where fertilization naturally occurs.
Uterus: hollow, muscular organ in which embryo implantation and fetal development occur.
Endometrium (endometrial lining): inner, vascular layer of the uterus where embryo implantation occurs; its contents are shed during a menstrual cycle if implantation does not occur.
Cervix: lower portion of the uterus that connects to the vagina. The cervix secretes cervical mucus.
Vagina: canal that runs from the cervix to the outside of the body.
Clitoris (not pictured): female equivalent to the male penis located on the outside of the vagina (known as the vulva).
Women are born with all of the eggs that they will ever have.
During fetal development (before birth), millions of eggs are produced in a female's ovaries, which remain inside the lower abdomen (unlike the testes, which descend into the scrotum). These eggs sit quietly inside the ovaries inside little sacs called follicles until puberty occurs. The first menstrual cycle is called menarche.
Most women remember how much puberty sucked. I know I do. And even though we're older now, we still have to deal with our periods at some point or another. But, as annoying as they are, our menstrual cycles are actually pretty amazing things. Our bodies are preparing for a pregnancy every. single. month.
Most menstrual cycles are ~28 days, so I'm going to use that number moving forward. Women continue to naturally have menstrual cycles until they reach menopause at approximately 50 years of age. Certain medications, such as birth control, can temporarily decrease or even prevent menstrual cycles from occurring.
A woman's menstrual cycle begins on the first day of menses (her period), which lasts around 4 days. Once the bleeding subsides, her body goes to work preparing for another pregnancy.
Over the next few days, three main events occur:
The ovary releases 1 egg from the dominant follicle, which is sucked into the Fallopian tube like a vacuum. The Fallopian tubes have fingerlike projections called fimbriae on their ends that sweep over the ovaries and catch the egg once it is ovulated.
The egg moves down the Fallopian tube for a few days, during which time fertilization can occur, and eventually rolls into the uterus. As this is occurring, three other things are happening:
As the menstrual cycle comes to an end, two possible events can occur:
Let's break it all down assuming that a new menstrual cycle has just begun:
Embryo implantation has not occurred, so the endometrial lining sheds its contents and the woman gets her period. In the ovary, the corpus luteum degenerates and stops producing progesterone and estrogen.
The brain detects the low levels of progesterone and estrogen and realizes that embryo implantation has not occurred. In response, the brain begins to produce GnRH, which causes the secretion of FSH and LH from the brain (like in males). The FSH travels to the ovaries and stimulates the "recruitment" of ~10-20 follicles (and their eggs) to mature. LH is also produced to help with follicle stimulation, just to a lesser extent.
As the follicles mature and a dominant follicle is chosen, the ovaries release large amounts of estrogen and inhibin. The high levels of estrogen cause the endometrial lining to thicken with blood and tissue to prepare for potential embryo implantation. It also causes the cervical mucus to thin out so that sperm can swim into the uterus and Fallopian tube(s). The inhibin eventually causes the brain to stop making FSH so no more follicles are stimulated in the ovaries.
After a few days, there is a really high level of estrogen and inhibin in the body. Once the estrogen reaches a certain level, a surge of LH is released by the brain, which triggers ovulation to occur. The inhibin quickly causes the brain to stop producing LH.
Ovulation occurs (the dominant follicle in the ovary releases its egg) after the LH surge.
The dominant follicle remaining inside the ovary becomes the corpus luteum, which secretes progesterone (and some estrogen). The progesterone causes the endometrial lining to continue thickening as the egg (or embryo) moves down the Fallopian tube and into the uterus. There is less estrogen being produced, so the cervical mucus thickens to block sperm transport into the uterus.
If implantation does not occur, the corpus luteum degenerates and no more progesterone/estrogen are produced. The endometrial lining sloughs off, causing a period to occur. The brain detects the low levels of progesterone/estrogen, and the cycle begins again.
If implantation does occur, the corpus luteum continues to produce progesterone/estrogen. This causes the endometrium to not shed its contents and also ensures that GnRH, FSH, and LH are not secreted the following month. The corpus luteum continues to secrete progesterone until the placenta takes over later in pregnancy. Once the embryo implants, it begins to secrete hCG (human chorionic gonadotropin), which can be detected with a pregnancy test.