The success of IVF treatment actually depends on how well a journey is planned. This journey takes place on the complex yet equally fascinating anatomical map of the female reproductive organs. The better we know this map, in other words, the better we understand the structure, location, and functioning of the organs, the more firmly we take each step of the treatment and the more easily we overcome the obstacles that may arise. Therefore, understanding these anatomical structures forms the foundation of the treatment.
What is the uterus and where is it located in the body?
The uterus is a pear-shaped, hollow organ made of very strong muscles, designed to host a baby throughout pregnancy. Its location in our body is quite central; it is safely positioned right in the middle of the female pelvis, just behind the urinary bladder and in front of the rectum, which is the final part of the large intestine. It is almost at the most protected point of the body.
We can think of this organ like a house. It has different chambers and sections from top to bottom. The wide and dome-shaped part at the very top is called the fundus. This is usually the area where the baby is most often located throughout pregnancy. The largest part that forms the main body is the corpus. Just below the corpus is the isthmus, a narrow section that provides the transition to the cervix. At the very bottom is the cervix, that is, the opening of the uterus into the vagina.
The size and shape of the uterus vary according to a woman’s stage of life. In a woman who has not yet given birth, it is approximately 8 cm long and 5 cm wide, while it may grow somewhat after births. With menopause, it tends to shrink again.
The position of the uterus is not the same in every woman. In most women (around 80%), the uterus is in a position slightly tilted forward toward the urinary bladder; this is called the anteverted position. In some women, on the contrary, it may be tilted backward, which is called retroverted. These positional differences are not a normal health problem and do not affect daily life. However, in IVF treatment, this position becomes important, especially in delicate procedures such as embryo transfer. For example, embryo transfer may be slightly more challenging in a uterus that is excessively tilted forward. In such cases, having a full urinary bladder before the transfer is a simple but effective method that corrects the angle between the uterus and cervix and makes the passage of the catheter easier.
What is the wall structure of the uterus like and why is it so important in IVF?
The uterine wall consists of three different layers from outside to inside, and each layer has its own unique, vital function:
On the outside, there is the thin perimetrium layer that surrounds the uterus like a membrane:
In the middle is the myometrium, the thickest and strongest layer of the uterus. This layer consists of smooth muscle fibers responsible for the powerful contractions that push the baby out during birth. In IVF treatment, however, it is very important for us that the myometrium remains calm. If this muscle layer is stimulated during embryo transfer, unwanted contractions may occur. Since these contractions may cause the newly placed embryo to be expelled from the uterus, the aim is to perform the transfer extremely gently and without trauma.
On the inside is the endometrium, the mucosal layer where pregnancy begins and the embryo attaches. The endometrium is one of the most critical players in IVF treatment.
The endometrium itself consists of two sublayers. We can compare this to a garden. The deeper stratum basalis layer is like the permanent soil that keeps the garden fertile at all times. The stratum functionalis layer on the surface is like plants that grow again every month, bloom, and are shed with menstrual bleeding. The embryo attaches to this fertile surface layer, which is prepared again every month. The health, thickness, and ability of this layer to accept the embryo are among the most important factors determining whether the treatment will be successful.
What are the differences between the uterine lining in the natural cycle and in IVF treatment?
In a natural menstrual cycle, the uterine lining (endometrium) changes under the guidance of a magnificent symphony directed by hormones. In the first half of the cycle, the estrogen hormone secreted from the ovaries takes the stage and allows the endometrium to thicken. Just as a gardener prepares the soil for planting, estrogen prepares the uterine lining for pregnancy. After ovulation occurs, progesterone, the second main hormone, comes into play. Progesterone changes the structure of this thickened layer; by enriching the blood vessels and enabling the glands to work, it transforms the endometrium into a soft and nourishing bed where the embryo can settle comfortably.
In IVF treatment, however, high doses of hormones are used externally to stimulate the ovaries and obtain a large number of eggs. This completely changes the hormone balance in the natural cycle. Estrogen and progesterone levels in the body rise far above normal. These high hormone levels cause the endometrium to mature much faster than normal. We can explain this situation with a train station analogy: In the natural cycle, the embryo (passenger) and the period when the uterus is ready (train) meet at the station at the same time. However, due to the high hormones in IVF treatment, the uterus (train) may leave the station early. When the embryo (passenger) is transferred into the uterus, that short and valuable “implantation window” during which the uterus is most suitable to accept the embryo may have been missed. This hormonal mismatch is one of the main reasons why pregnancy rates in fresh embryo transfers can sometimes be lower than expected. Therefore, freezing the embryos and transferring them in the following month, when the uterus is prepared in a more natural hormonal environment (frozen embryo transfer), is an important strategy that can increase the chance of success.
How does the blood circulation of the uterus affect the chance of pregnancy?
For the uterus to host a healthy pregnancy, it must have excellent blood circulation. Just as a fertile field needs good irrigation, the endometrium also needs abundant blood flow to nourish the embryo. The uterus is nourished by a rich blood network coming from both its own arteries and the ovarian arteries. These main vessels advance deep into the uterine wall and ultimately form the tiny spiral arteries that nourish the endometrium and are vital for pregnancy.
The healthy development of this vascular network is critical for the endometrium to reach sufficient thickness and for the embryo to continue developing after attaching to the uterus. Insufficient blood flow may cause the endometrium not to develop adequately and may reduce the chance of embryo implantation. During the treatment process, blood flow in the uterus can be measured with a special ultrasound technique called Doppler ultrasound. These measurements give us valuable clues about how ready the uterus is to accept the embryo and may help us shape our treatment strategy accordingly.
How do fibroids and polyps in the uterus prevent pregnancy?
Fibroids and polyps are benign formations frequently encountered in the uterus and may significantly affect the chance of pregnancy depending on their location and size. The effects of fibroids on pregnancy are basically divided into three groups.
- Submucosal fibroids
- Intramural fibroids
- Subserosal fibroids
Submucosal fibroids grow into the uterine cavity, that is, toward the area where the embryo will settle. Therefore, they are the most problematic. By distorting the space inside the uterus, like an unnecessary object placed in the most beautiful room of the house, they mechanically prevent the embryo from settling. It is generally recommended that these types of fibroids be removed before IVF treatment with a closed surgery called hysteroscopy.
Intramural fibroids are located within the muscle layer of the uterine wall. If they are very large or exert pressure on the uterine cavity, they may make embryo implantation more difficult by disrupting blood flow in the area or causing contractions in the uterus.
Endometrial polyps are finger-like protrusions arising from the uterine lining and extending toward the uterine cavity. Just like submucosal fibroids, they may create a mechanical obstacle, cause a constant mild inflammatory reaction inside the uterus, and secrete some chemical substances that prevent embryo implantation. Removing polyps, usually by hysteroscopy, makes the uterus more suitable for pregnancy.
What are the ovaries and where are they located in the pelvis?
The ovaries are a pair of almond-shaped organs where the egg cells (oocytes), which are the source of a woman’s reproductive potential, are stored and mature. They are located on both sides of the uterus, in the pelvic cavity. Their location is quite strategic; they are very close to the large blood vessels and to the channels called ureters, which carry urine from the kidneys to the bladder. This anatomical proximity is of great importance especially during the egg collection (OPU) stage of IVF treatment. During this procedure, when the ovaries are reached with a needle under ultrasound guidance, great care must be taken not to damage these neighboring structures.
The ovary consists of the cortex layer on the outside, which contains the egg sacs (follicles), and the medulla layer on the inside, which contains blood vessels and nerves. Each month, one of the follicles in this cortex layer grows and prepares for ovulation.
What kind of process is egg development and maturation?
A baby girl is born with all the egg precursors she will use throughout her life. These millions of egg precursors are stored inside tiny dormant sacs called primordial follicles. With puberty, every month a group of these sleeping follicles wakes up and begins a long and complex maturation journey. This process progresses under the sensitive control of hormones, and its stages are as follows:
- Primordial follicle
- Primary follicle
- Secondary follicle
- Antral follicle
- Graafian (dominant) follicle
During this process, as the follicle grows, the egg inside it also matures. When the antral follicle stage is reached, a fluid-filled cavity forms inside it, and growth after this stage comes under the control of the FSH and LH hormones secreted from the brain. In IVF treatment, thanks to the externally administered FSH hormone, many of these antral follicles, normally only one of which would grow each month, are made to grow at the same time.
Among these follicles that reach sufficient size, the dominant follicle called the Graafian follicle becomes ready for ovulation. In IVF treatment, however, when the follicles reach the desired size, an hCG (trigger shot) injection is administered so that the eggs complete the final maturation step. This injection mimics the effect of the LH hormone secreted from the brain in the natural cycle, and the egg collection procedure is planned approximately 34-36 hours after this injection.
How does ovarian blood circulation affect IVF treatment?
The ovaries are organs with very rich blood supply, nourished from two different sources: both their own arteries coming directly from the main abdominal artery (aorta) and through a branch coming from the uterine artery. This strong blood circulation is vital for the healthy functioning of the ovaries. In IVF treatment, this blood flow must be sufficient for the hormone medications (gonadotropins) we administer externally to reach the growing follicles. Insufficient blood flow may reduce the ovaries’ response to medications, meaning it may cause fewer eggs to develop. The decrease in ovarian blood vessel density with age is also an important cause of the decline in female fertility. Evaluating ovarian blood flow with Doppler ultrasound may help us predict the patient’s ovarian response before starting treatment and adjust the medication dose more accurately.
What are the fallopian tubes and why are blocked tubes a problem?
The fallopian tubes are thin, tube-shaped structures extending from both corners of the uterus toward the ovaries. For a pregnancy to occur naturally, the tubes must be open and healthy. Because the egg released from the ovary is captured by the tube, fertilization with the sperm takes place inside the tube, and the resulting embryo makes its journey toward the uterus through these tubes.
In IVF treatment, since the egg is fertilized outside the body and the embryo is placed directly into the uterus, this transport function of the tubes is not needed. However, if the tubes themselves are diseased, especially if fluid has accumulated inside them (hydrosalpinx), this seriously affects treatment negatively. Hydrosalpinx usually occurs as a result of blockage of the end of the tube due to previous infections. This fluid accumulated inside the tube is not an innocent fluid. On the contrary, it contains substances toxic to the embryo (embryotoxic) and inflammatory cells. Since the end of the blocked tube that opens into the uterus is usually open, this harmful fluid may flow back into the uterus. The negative effects of this situation are as follows:
- Creating a direct toxic effect on the embryo
- Mechanically washing the embryo inside the uterus out
- Disrupting the ability of the uterine lining to accept the embryo
For these reasons, the presence of hydrosalpinx almost halves IVF pregnancy rates and increases the risk of miscarriage. Therefore, before starting IVF treatment, removing the tube with hydrosalpinx by laparoscopic (closed) surgery (salpingectomy) or closing it from the point where it connects to the uterus is the standard and most correct approach that restores the chance of success to normal.
What is the role of the cervix and vagina in IVF procedures?
The vagina and cervix are our gateway for two important procedures of IVF treatment: egg collection and embryo transfer. The anatomical structure of this route directly affects the ease and success of the procedures.
During the egg collection procedure, a needle attached to the ultrasound probe is passed through the back wall of the vagina and advanced to the ovaries. Therefore, the anatomy of the vagina serves as a passage for this procedure.
Embryo transfer depends entirely on the condition of the cervical canal. The cervix is a cylindrical structure approximately 2-3 cm long, opening the uterus into the vagina. This canal is not always like a perfectly straight tube. In some women, it may have mild or pronounced curves. In addition, due to previous infections or surgical procedures, there may be narrowing (cervical stenosis) in this canal. These anatomical differences may make it difficult for the thin and delicate catheter carrying the embryo to advance smoothly into the uterus. Any difficulty encountered during the transfer may reduce the chance of embryo implantation by causing trauma, bleeding in the cervix, and uterine contraction. Therefore, it is very important to know the structure of the cervical canal in advance.
How are anatomical difficulties encountered in embryo transfer managed?
It has been scientifically proven that a “difficult” embryo transfer reduces pregnancy rates. A difficult transfer is not only a technical problem but may also trigger a physiological reaction that prevents embryo implantation. Therefore, we have various strategies to predict and manage possible anatomical difficulties. These strategies are as follows:
- Performing a mock transfer before the procedure
- Relieving cervical stenosis (dilation)
- Managing the sharp angle (full bladder, use of tenaculum)
- Choosing an appropriate and personalized catheter
One of the most valuable methods is performing a “mock transfer” in a period before the actual transfer. During this procedure, an empty catheter is passed through the cervical canal, and the length, direction, possible curves, or narrowings of the canal are detected in advance. Thus, when the actual transfer day arrives, which catheter will be used and what route will be followed are planned in advance. If a narrowing is detected, this area can be gently widened before the transfer. If there is a sharp uterine angle, it is possible to straighten this angle with a full urinary bladder or by gently pulling the cervix with a special instrument (tenaculum). Since every patient’s anatomy is different, choosing the most suitable catheter and performing the procedure under ultrasound guidance in the gentlest way is the key to success.
How do congenital uterine anomalies form in the womb?
The uterus, fallopian tubes, and upper part of the vagina originate from a pair of structures called Müllerian ducts during development in the womb. This development takes place in three basic stages: first, these two ducts form, then they approach each other in the midline and fuse, and finally the middle wall at the site where they fuse dissolves, forming a single uterine cavity. A disruption occurring at any of these stages leads to congenital uterine anomalies. An important point is that the ovaries develop from a completely different structure than this system. Therefore, the ovaries and hormones of women with a congenital anomaly related to the uterus are usually completely normal. In other words, although there may be no problem with egg production, there may be a structural problem in the home where the embryo will settle.
What are the most common uterine anomalies and how do they affect pregnancy?
Congenital uterine anomalies may significantly affect pregnancy outcomes. The most common types and their effects are as follows:
Uterine Septum (Septate Uterus): It is the most common anomaly. It occurs when the wall (septum) between the Müllerian ducts does not completely dissolve after they fuse. This septum is a tissue quite poor in terms of blood circulation. If the embryo implants on this septum, it cannot be nourished sufficiently and the pregnancy usually ends in miscarriage. It is one of the important causes of recurrent pregnancy loss. Its treatment is removing this septum by cutting it with hysteroscopy (metroplasty). This simple and effective procedure dramatically improves pregnancy outcomes.
- Developmental Error: Failure of the middle wall to dissolve
- Appearance: Externally normal, internally septate uterus
- Effect: High risk of miscarriage and implantation failure
- Management: Hysteroscopic septum incision
Bicornuate Uterus (Heart-Shaped Uterus): It is the formation of two separate horns in the upper part of the uterus in a heart shape as a result of incomplete fusion of the ducts. This condition may cause the internal volume of the uterus to be narrower and uterine contractions to be abnormal. It may increase risks such as miscarriage, preterm birth, and the baby being in a breech position in the uterus.
Unicornuate Uterus (Half Uterus): It is the uterus being half the normal size as a result of the development of only one of the Müllerian ducts. Since uterine capacity is reduced, the risk of preterm birth and miscarriage is quite high.
Uterus Didelphys (Double Uterus): It is the presence of two separate uteri and usually two separate cervixes as a result of the ducts not fusing at all. Just as in bicornuate uterus, the risk of preterm birth is increased.
Three-dimensional ultrasound and MRI (Magnetic Resonance Imaging) are the gold standard methods in the diagnosis of these anomalies. The treatment plan is planned individually according to the type of anomaly and the patient’s history.
