Uterus – Anatomy, Function, And Pregnancy

The uterus is one of the most important organs in the female body because, without it, human life would not even be possible. The embryo develops in the uterus until birth. Here you will learn what the uterus looks like and how it is structured, what its tasks are and what happens to the uterus during pregnancy.

The uterus is one of the most fascinating organs of the female body. During pregnancy, it changes and adapts to the growing embryo. It is responsible for the care of the unborn child.

The mucous membrane is also adapted to the function of the uterus: It is built up high during the menstrual cycle to accommodate a fertilized egg if necessary. If fertilization and implantation do not occur, the uterus sheds the upper layers of the mucosa with menstrual bleeding.

During birth, the muscles of the uterus play a special role: they contract rhythmically and thus push the baby through the birth canal. Furthermore, the retaining ligaments of the uterus are also designed to adapt to its strong growth.

Even though the most common conditions, such as fibroids, do not cause serious health problems, you should not miss the offer of regular check-ups.

What Is The Function Of The Uterus?

The actual function of the uterus comes into play only during pregnancy. In the uterus, the fertilized egg develops into a viable child.

Every month the uterus prepares itself for exactly this task. The uterine lining thickens to a thickness of about six millimeters in the first half of the cycle under the influence of hormones.

During the fertile phase of life, the endometrium is subject to a monthly recurring build-up, remodeling, and partial breakdown of the endometrium, which is controlled by the sex hormones estrogen and progesterone. If fertilization of the egg has not occurred, the cycle repeats. It takes place in three phases:

  • Menstrual phase: after the corpus luteum has regressed and the production of the corpus luteum hormone (progesterone) has ceased during the cycle that has elapsed, the newly formed endometrium receives less blood supply and is shed with the menstrual or period bleeding. 50 to 150 milliliters of blood, tissue remnants, and mucus are expelled. During this process, the strong muscle layer inside the uterus contracts to expel the rejected tissue to the outside. These muscle contractions cause menstrual pain, which can vary in intensity from woman to woman.
  • Reconstruction phase (follicular phase): the lining of the uterus is rebuilt, stimulated by the hormone estrogen, which is secreted by the follicle in increasing concentrations.
  • Secretion phase (luteal phase): After ovulation in the middle of the cycle, the corpus luteum matures in the ovary, producing the hormones estrogen and especially progesterone (corpus luteum hormone). The hormones cause the uterine lining to thicken further and the blood vessels to branch out, and nutrients are secreted into the uterine cavity.

From the fourth day after ovulation, the uterine lining is prepared for implantation of a fertilized egg. If it fails to do so, the corpus luteum regresses and stops producing hormones.

However, if an egg is fertilized within a cycle, it nests in the lining of the uterus. This causes it to continue growing to provide nourishment for the embryo. The uterus adapts to the growth of the baby.

What Happens To The Uterus During Pregnancy?

When a fertilized egg nests in the lining of the uterus, it signals the ovaries to continue releasing progesterone. As a result, the mucous membrane is not rejected.

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The placenta, also called the placenta, develops from an attachment to the egg. It is an independent organ that acts as a barrier separating the blood of the mother and embryo. At the same time, it allows nutrients and oxygen to pass through, supplying the offspring.

As the baby grows, the uterus grows and stretches. Towards the end of pregnancy, as already mentioned, it has grown by quite a bit and is about 20 times larger than it was at the beginning of pregnancy.

During labor , the uterus contracts and contractions occur. They help to expel the baby from the womb. At the end of pregnancy, the uterus usually returns to approximately its original size within about six weeks.

After Implantation

Immediately after implantation of the fertilized egg, the uterus begins to enlarge. However, the uterus is usually not noticed until the end of the first trimester (1st to 13th week of pregnancy), when it grows beyond the pubic symphysis.

Some women feel a sense of heaviness in the abdomen at a very early stage, as well as pain in the pelvic area that resembles menstrual cramps and is caused by increased blood flow as well as the loosening of uterine tissue and stretching of the uterine ligaments. The uterine muscles also thicken, therefore by the end of the first trimester your waist circumference has already increased.

It is not uncommon to find fibroids of the uterine wall during ultrasound examinations during pregnancy, but they usually do not have a negative effect on the course of pregnancy.

In The Second Trimester

From the 16th week of pregnancy, when the enlarged uterus grows beyond the pubic symphysis, many women can already tell that they are pregnant. This depends on various factors, such as how many times the woman has been pregnant before and how big the baby is at this point. Body size and build also play a role, which is why small and petite women look pregnant earlier.

In The Last Trimester Of Pregnancy

By the end of the second or beginning of the third trimester, a pregnant woman’s girth has already increased so much that sleeping problems occur, and lying on your back is no longer possible.

In addition, the pressure of the uterus causes many women’s bellybuttons to bulge out somewhat, but this disappears again after the birth. In addition, short, painless contractions of the uterus are increasingly felt at this stage, so-called pre-labor contractions, which promote blood flow and prepare the body for real labor.

The Size Of The Uterus At Different Stages Of Pregnancy

4.SSW: Uterus is normal size8.SSW: Uterus is as big as a goose egg12.Week 16: Uterus is as big as a fist16.Week 16: uterus reaches two transverse fingers above the pubic bone20.Week 20: upper edge of the uterus is three fingers below the navel 24.Week 28: upper edge of the uterus is at the level of the navel 28.Week 32: upper uterine margin is three transverse fingers above the navel 32.Week36: upper uterine margin is between the navel and the sternum36. 40thweek: upper uterine margin is at the costal arch40. 40thweek: the upper edge of the uterus is two transverse fingers below the costal arch.

The Uterus After Birth

What happens to the uterus after birth? The period after birth until the uterus has fully regressed and hormone levels have returned to pre-pregnancy levels is called the postpartum period (puerperium). This period lasts six to eight weeks.

During the period of involution, the uterus regularly contracts. These so-called afterpains can begin immediately after birth and usually last a few days. The pain in labor after birth is usually mild. It usually increases after the birth of more children, because the already pre-expanded uterus has to contract more.

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Placing the baby against the mother’s breast also causes the uterus to contract more through a release of hormones. If the pain becomes very uncomfortable, a mild painkiller can provide relief after consultation with the attending physician.

The reduction in size of the uterus also causes the inner wound area at the attachment site of the placenta to shrink. A sign that the wound is healing is the postpartum flow (the lochia). This secretion consists of tissue parts and blood and is always colonized by bacteria.

The quantity, as well as the color and odor of the lochia, change during the postpartum period. Initially, they are predominantly bloody red, then the color becomes brownish, and later yellowish to whitish. Overall, lochia lasts for about four to six weeks.

Pelvic floor training

Postpartum exercises help the body to return to its original state after pregnancy. The focus is on training the pelvic floor muscles. In the short term, this gymnastics promotes the elimination of postpartum fluid, stimulates the often rather sluggish bowel activity and promotes purification. In the long term, it prevents a lowering of the pelvic floor with possible subsequent involuntary leakage of urine, uterine prolapse and back pain.

The woman can begin simple exercises as early as the day of birth, even after a cesarean section. Instruction is usually provided by specially trained physiotherapists at the maternity hospital or by midwives. It is important to perform these exercises for at least a few weeks so that the heavily stressed muscles can strengthen again.

Anatomy Of The Uterus

Where Is The Uterus Located?

The uterus is located in the human body in the center of the female abdomen. The anterior part lies on the urinary bladder, while the posterior part is adjacent to the rectum. The uterus and its neighboring organs are covered by the peritoneum, creating peritoneal pits between each organ. The exact location of the uterus depends on how the rectum and urinary bladder are.

The uterus is normally slightly bent forward (anteversion) and slightly bent forward relative to the cervix (anteflexion). It rests on the urinary bladder in this way. Depending on the filling of the urinary bladder, the uterus shifts a little.

The anchoring of the uterus in the female pelvis is ensured, among other things, by the so-called round uterine ligaments, a ligament running towards the pelvic wall, and by the so-called parametrium. This is a frontal plate made of connective tissue and reinforced by several ligaments. These ligaments hold the uterus in a flexible floating position supported by the pelvic muscles.

Appearance And Structure Of The Uterus

The uterus is a hollow organ that is often compared to an upside-down pear. Its weight varies from 30 to 120 grams.

Typically, the uterus is about seven centimeters long, three centimeters thick and five centimeters wide, but the size varies from woman to woman. Just before a woman gives birth, the uterus grows and reaches the size of two small footballs, while the weight of the muscle mass is up to 500 grams. The uterus is the only organ capable of such massive changes.

The Structure Is Divided Into Three Parts:

  • Perimetrium: outer lining with peritoneum.
  • Myometrium: thick middle layer of smooth muscle cells.
  • Endometrium: inner mucous membrane lining the uterine cavity.

The cervix forms the lower part of the organ, known in technical jargon as the cervix uteri. This encloses the cervical canal, the canalis cervicis, and is the connection between the vagina and the uterus.

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The canal begins with the outer cervix, which curves into the vagina, and ends in the inner cervix. This inner part is by far the narrowest part of the cervical canal, with a diameter of about three millimeters. At the upper ends, the uterus closes off and merges with the fallopian tubes.

To fulfill its function as a barrier between the vagina and the inner part of the uterus, the cervix secretes mucus. The consistency of the mucus changes during the cycle.

On infertile days, it is viscous and seals the cervix so that neither bacteria nor sperm can pass through. On fertile days, on the other hand, i.e. a few days before and after ovulation, the mucus is thin. Then sperm can rise into the cervix and enter the uterus to fertilize an egg.

The Lining Of The Uterus

The inner layer of the uterine wall forms a pinkish mucous membrane, also called the endometrium. The endometrium is always renewing itself.

During the menstrual cycle, the lining of the uterus changes. If the woman is of procreative age, part of the endometrium is shed at regular intervals (unless fertilization of the egg has occurred) – menstrual bleeding occurs.

This process is repeated again and again, the exception being pregnancy. If a fertilized egg has been implanted, the endometrium contributes to the formation of the placenta. In this case, it is expelled after birth together with the placenta as afterbirth.

Function Of The Endometrium

The endometrium serves to protect the uterus. The mucosal cells ensure that the uterus cannot be damaged quickly and that the organ is safe. After puberty, female hormones begin to work in a woman’s body. These control the woman’s cycle and prepare the body for possible fertilization. After menstruation, the tissue of the uterus changes – sex hormones then influence the thickness and size of the mucous membrane to prepare it optimally for the egg.

Approximately once a month a part of the uterine lining is shed, this process is also called menstruation. However, only the functional layer of the mucous membrane is shed, but not the protective basal layer, which is located above the uterine muscles.

The first menstrual period is called ‘menarche’ and the last is called ‘menopause’. If an egg has implanted in the uterine mucosa after fertilization, menstruation stops and pregnancy occurs. During pregnancy, the mucous membrane is permeated by many blood vessels and glands to supply the egg with enough nutrients.

Fallopian Tubes

The two fallopian tubes extend to the left and right from the upper end of the uterus. They are between ten and 18 centimeters long, pencil-thick, and are the connection between the uterus and ovaries. The funnel-shaped ends of the fallopian tubes lay close to the respective ovary in the uterus to receive the egg. The actual fertilization takes place in the fallopian tube.

After fertilization, the egg continues to travel for about three to five days and usually nests in the lining of the uterus on the sixth day of fertilization. If implantation already occurs in the fallopian tube, it is called an ectopic pregnancy. If implantation occurs in the abdomen, it is called an abdominal pregnancy.

Ovaries

The two ovaries are located on the right and left sides of the pelvis near the ends of the fallopian tubes. The ovaries are where the immature eggs are stored. In each ovary, there are countless clusters of cells called follicles in which eggs mature. At ovulation, the follicle breaks open and releases the mature egg into the fallopian tube.

As early as the 20th week of pregnancy, a female embryo has 8 million eggs (primary follicles); at birth, there are 1-2 million, which reduces to about 300,000 to 500,000 by puberty. In a woman’s lifetime, there are about 300 to 500 cycles of ovulation.

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In addition, the sex hormones estrogen and progesterone are formed in other cells of the ovaries. Due to the regulated formation of the hormones, at least one egg normally matures in the ovaries every month with the first menstrual bleeding, which is then released during ovulation.

Possible Complaints And Diseases

Myomas

A great many women have benign growths in or on the uterus called fibroids. However, depending on where they form, they can cause period pain, heavy menstrual bleeding, and other symptoms.

Fibroids are made up of muscle cells and connective tissue. They vary in size, shape and location. Myomas are divided mainly according to where in the uterus they are located:

  • Directly under the lining of the uterus (submucosal fibroids).
  • In the uterine wall (intramural fibroids).
  • On the outer side of the uterus (subserosal fibroids).
  • On the cervix (cervical myoma).
  • In the connective tissue at the side of the uterus (intraligamentous myoma).

Many women do not even feel that they have fibroids. Myomas usually only need to be treated if they cause discomfort, affect fertility, or could lead to problems during pregnancy.

Uterine Prolapse

The uterus is supported by the muscles of the pelvic floor. When the muscles are weak and give way, the uterus descends downward into the vagina. To prevent uterine descent, pelvic floor exercises and maintaining normal body weight, i.e. reducing excess weight, are helpful. Regular exercise is a very good way to prevent prolapse.

Uterus removal

Uterus removal, also called hysterectomy , is one of the most common surgical procedures in gynecology. There are many different reasons for uterus removal. In about ninety percent of cases, they are benign conditions that can cause severe menstrual cramps, for example. These include:

Muscular growths of the uterus (fibroids)- Endometriosis – a descent or prolapse of the uterus.

Endometriosis

Endometriosis refers to endometrium, the lining of the uterus that settles outside the uterus. It is considered the second most common female disease, the causes of which are still largely unexplored. The symptoms can be alleviated with certain hormone preparations, among other things.

The symptoms usually begin between the ages of 20 and 30, often with the first menstrual period. How endometriosis manifests itself varies from woman to woman.

Endometriosis often progresses without any symptoms. However, if symptoms do occur, they are usually cramps and persistent pain in the lower abdomen during menstruation. Back pain can also be a sign of endometriosis, as can pain during sexual intercourse. As a result of unchecked mucosal growth, fertility may be limited.

Uteritis

Uterine inflammation can spread to different layers of the organ. An important indicator of cervicitis is purulent discharge (genital fluoride). The discharge is often increased, yellowish-white and foul-smelling. Since cervicitis is usually preceded by an infection of the vagina, there is often additional itching and burning of the vagina.

Uterine inflammation always occurs when the natural protective mechanisms of the organ are damaged. Accordingly, various triggers come into consideration. Among the most common causes are the consequences of childbirth.

If you suspect that you are suffering from a disease of the uterus, it is best to visit your gynecologist for clarification so as not to worry unnecessarily. In general, regular check-ups are important and help to detect and treat diseases at an early stage.

Sources

  • Frank H. Netter. Gynecology, Thieme (2006).
  • Goerke, Valet: Brief Textbook of Gynecology and Obstetrics, Urban And Fischer, 7th ed.
  • Goerke, Steller, Valet: Clinical Guide Gynecology Obstetrics, Urban And Fischer, 10th edition.
  • Kaufmann, Costa, Scharl. Gynecology, Springer, 2nd ed.

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