Basic Reproductive Anatomy
The uterus is a muscular organ that lies within the pelvis between the bladder and rectum. It is shaped like upside-down pear and is about the size of a fist in a non-pregnant woman. The uterus is composed of the fundus (dome shaped portion above the tubes), the body (largest tapering central portion) and the cervix (opening into the vagina).
If a woman inserts a speculum into her vagina and opens it, she will see her (beautiful!) cervix, which looks like a little pink doughnut about 3-6 inches from the opening her vagina. The cervix is the lowermost part of the uterus that extends into the vagina and connects the uterus to the top of the vaginal wall (that’s right – the vagina is a sealed cavity and doesn’t open into the abdomen – you can’t lose a tampon or condom within it). The cervix opens to the vagina via an internal os and an external os, though the external os is the only one visible upon cervical self-exam – it is the little hole in the center of the doughnut. The inner canal of the cervix, which you also can’t see upon speculum exam, is lined with glands that produce secretions that vary in consistency and quality throughout the cycle. The quality (color, consistency, texture) of this fluid gives us accurate information about our current fertility or possibly an infection.
The uterus is made of three layers of tissue. The outer layer is called the perimetrium (or serosa) which becomes the broad ligament on either side (shown as the yellow sheet-like structure in the image above). The bulky middle layer of the uterus, called the myometrium, is composed of three muscle layers, which are thickest at the fundus and thinnest at the cervix. Pound for pound, the uterus is the strongest muscle in the female body. Amazingly, the thin muscle of cervix thins and dilates with the rhythmic contractions of labor allowing the os to open to 10 cm (much like a head being pushed through a tight turtleneck shirt). The incredibly powerful muscles of the fundus push the baby from within the uterus into the vagina during labor and as it is birthed through the vulva. The innermost lining of the uterus is called the endometrium and it creates a thick, specialized tissue each cycle that is shed during menstruation. It is this velvety nourishing tissue that becomes the site where the placenta grows during pregnancy to nourish the fetus.
Branching from the top of the uterus, there are two oviducts (also called uterine or Fallopian tubes) that open to body of the uterus. The oviducts are muscular passageways that help sweep the egg from the ovary to the uterus at ovulation. At the end of the oviducts are finger-like structures called fimbriae, which move over the surface of the ovary to engulf the egg released during ovulation- this looks like a sea anemone moving in the ocean.
The fimbriae nestle two ovaries, which are organs about the size of almonds. Each ovary holds many follicles, or tiny sacks that contain immature eggs, which are not visible to the naked eye.
It is estimated that women are born with about 1-3 million immature eggs, called oocytes or follicles, that live within the ovaries. Unlike a man who produces his sex cells (sperm) every day, a woman is born with a certain number of sex cells that gradually die over time beginning in infancy and continuing through menopause. When a woman first gets her period at puberty, only about 400,000 follicles remain in her ovaries. With each menstrual cycle, a thousand follicles are lost and (usually) only one follicle will actually mature into an ovum (egg), which is released into the oviduct, marking ovulation. Depending on how many of her reproductive years are spent pregnant or not ovulating due breastfeeding, it is estimated that between only about 400 of the original 1-3 million will ever mature into ova.
The menstrual cycle is a complex interaction of the six key hormones (chemical messengers) that affect changes in the reproductive organs (uterus, ovaries, breasts, and oviducts). It is divided into three distinct phases: the follicular, ovulation, and the luteal phases.
The length of a menstrual cycle – counted from the beginning of one menstrual period to the beginning of the next – can be different for each woman or change from cycle to cycle for the same woman. You will see that many menstrual cycle diagrams represent a 28 day cycle, which is an average, but a healthy menstrual cycle can vary between 21-35 days.
The menstrual cycle begins when a woman begins bleeding, often called getting her period. The menstrual fluid contains blood, cervical mucus, vaginal secretions, and endometrial tissue, though most people just refer to it as blood. At this point, estrogen and progesterone are the lowest they will be throughout the cycle. Menstruation marks the beginning of the follicular phase, which ends at ovulation. The length of the follicular phase is variable and can be affected by diet, stress, or illness.
Triggered by a complex series of chemical signals, the pituitary gland in the brain begins to secrete two key hormones, Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH). The increase in FSH and LH cause about 10-20 of the follicles within the ovaries to begin to mature and they too release a hormone, called estrogen or estradiol. Levels of estrogen in the body steadily increase as the follicles continue to mature for about a week – this estrogen causes the lining of the uterus to begin to thicken. Usually only one of the maturing follicles becomes dominant, as it is most sensitive to estrogen, LH, and FSH. When the estrogen level reaches a certain threshold, it triggers the pituitary gland to release a large amount of LH.
Cervical fluid in the follicular phase is typically scant or tacky. For many women who are tracking cervical position, the cervix feels hard, closed, and low. As ovulation approaches, the cervical fluid becomes more watery and lubricative/slippery and stretchy like raw eggwhites and the cervix begins to soften.
The spike in LH (seen as the peak in the green line on the graph below) cause the fully mature dominant follicle to burst through the wall of the ovary. The release of the mature ovum is called ovulation. Some women literally feel ovulation and experience ‘mittlesmertz,’ or a slight pain in one of their ovaries during ovulation.
The cervical mucous near ovulation is typically very stretchy, slippery, and clear (looks and feels like eggwhites). This fertile fluid helps the sperm move toward the egg and protects sperm from the acidity of the vagina, which would normally kill them. The cervix feels higher, softer, and more open.
The mature ovum released from the ovary at ovulation is swept by the fimbriae into the oviduct by rhythmic muscular movement of the oviduct, where it will live for about 24 hours if not fertilized by sperm. The remains of the dominant follicle in the ovary is called the corpus luteum and produces large amounts of progesterone during the luteal phase. The length of the luteal phase is not variable and typically lasts 14 days for most women, though a range of between 12-16 days is considered normal.
If conception or implantation does not occur, the corpus luteum in the ovary will shrivel about 14 days after ovulation and will cause a sharp decrease in both estrogen and progesterone, triggering the onset of menstruation and the beginning of a new menstrual cycle. The unfertilized ovum is about the size of a grain of sand; it is shed as part of the menstrual fluid.
Progesterone is high during the luteal phase; it is a heat inducing hormone and thus raises the basal body temperature (BBT) by several tenths of a degree for the duration of the cycle. The jump in basal body temperature that remains steady indicates a woman has ovulated.
The cervix feels low, firm, and closed during the luteal phase.
In order for conception to occur, semen must first survive the acidity of the vagina (fertile cervical fluid changes the vaginal pH to be more alkaline and sperm friendly) and then swim through the cervical canal into the body of the uterus and then into the oviduct to greet a woman’s egg. If an ovum is fertilized by sperm in the oviduct (called conception), the fertilized egg (blastocyst) migrates into the uterine lining, where it will implant approximately 7 to 14 days after ovulation. The corpus luteum will continue to provide high levels of progesterone to support pregnancy until the placenta takes over that job in approximately 12 weeks.
How does the cervix change throughout the cycle?
If you’re already charting your menstrual cycle, checking the position and texture of your cervix each day can help you confirm where you are in your cycle. If you’re not already charting, have a feel anyway – its a great skill to have in your empowered woman toolbox!
Here’s how to feel your cervix:
- Wash your hands
- Squat or stand with one foot raised on a stool.
- Insert your longest finger into your vagina until you feel your cervix. It will feel like a protruding nub/cylinder toward the back of the soft walls of your vagina. If your finger is long enough, you should be able to circle your finger all the way around the cervix and feel a little dent in the middle of it (called the os, the opening to the uterus).
- Note the following:
- How deep in your vagina is your cervix resting? (How much of your finger is inside of you?)
- Does your cervix feel soft, like pursed lips, or more firm, like the tip of your nose?
- Is your cervix angled to one side or aligned more centrally?
- Does your os feel slightly open and squishy or squeezed shut?
While menstruating, the cervix may feel firm and low and the os open as it releases blood. It may be angled to one side slightly. Once all the blood has been shed, the os again feels closed.
As ovulation nears, the rising levels of estrogen cause the ligaments that attach the uterus to the pelvis to tighten and pull the uterus up further into the body. Hence, the cervix gradually draws deeper in the vagina and if often harder to reach near ovulation. The cervix may feel softer (like pursed lips) be more centrally aligned, and the os slightly open.
After ovulation, estrogen levels drop and the cervix usually resumes feeling low, firm, and closed until she gets her period a few weeks later. It is not uncommon for the cervix to be tilted to one side.
Not all women follow this pattern exactly so don’t worry if you don’t. For example, women with retroverted (tipped) uteruses may find thier cervix easier to reach near ovulation and women who have given birth vaginally usually have softer cervices throughout the cycle. Its empowering just to know what is normal for you. You may notice your cervical changes vary from cycle to cycle or that you have a consistent pattern that aligns with your other symptoms of fertility (cervical fluid and basal body temperature).
The key is to check every day so you can feel the relative differences from day to day; cervical changes can be very subtle. Check your cervix in the same position and at the same time each day (i.e. in a squat before showering in the morning), so you’re comparing apples to apples – or cervices to cervices, as the case may be.
Men produce about 1000 sperm per second and are thus considered fertile everyday. A man’s ejaculate contains between 50-500 million sperm. Conception can only occur when one of these live sperm meets a woman’s live egg.
Women ovulate (release an egg) once per menstrual cycle and the egg lives a maximum of 24 hours. For a few days prior to ovulation, tiny glands in the cervix called cervical crypts produce fertile cervical fluid (a wet, often slippery, raw eggwhite consistency). This fertile cervical fluid can help sperm survive for up to five days in the vagina as they patiently wait for the egg to be released. So, even though her egg itself only lives for about a day or less, women are considered to have a ‘fertile window’ when they are producing fertile cervical fluid – meaning that intercourse/insemination in that window of time could lead to conception if the sperm stay alive (for a few hours or even days) in the fertile fluid and then make their way into the Fallopian tube to fertilize the egg after ovulation.
There are a variety of methodologies for calculating a woman’s natural fertility windows, both religious and secular – collectively they are called Fertility Awareness Methods (FAM) or Natural Family Planning (NFP). Some are less accurate and based on guessing a women’s fertility based on averages of past cycles (Calendar methods/the Rhythm Method/many period tracker Apps). Some are based in checking cervical fluid only (the Ovulation Method). A very effective practice is called the Symptothermal Method (STM) of FAM and is based on the scientific facts that hormonal fluxes during a woman’s menstrual cycle cause observable changes in the quality of her cervical fluid and a rise in her basal body temperature (BBT, or temperature upon waking) after ovulation.
In practice, STM is relatively simple: throughout the day, a woman notices and records the sensation and quality of her cervical fluid as it appears at her vulva or on her toilet paper after wiping. On this chart, she also records her basal body temperature, taken orally, vaginally or under her armpit when she first wakes up in the morning. Optionally, some women also check the firmness and depth of their cervix within their vagina to confirm the other two fertility signs on their charts.
STM takes about 4-8 hours to learn, either through reading a book, taking a workshop or during one-on-one lessons with a teacher/mentor. Once initial guidelines are understood, STM takes a commitment of about 2 minutes a day to maintain and interpret data on her chart.
STM can become an empowering path of enhancing a woman’s overall body literacy, increasing communication with her partner, and gauging her overall health. STM can be used as a natural form of birth control, to help achieve pregnancy, and to find underlying health issues that may be affecting the regularity of her cycles or her fertility overall.