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Your Uterus and IVF – How the Uterine Environment Affects Embryo Transfer Outcomes

All surrogates and IVF patients go through medical screening at the fertility clinic at the very beginning of the IVF process. One purpose of this screening is to thoroughly exam the uterus, in order to make sure there’s no uterine abnormalities that could negatively impact the embryo transfer.

While most surrogates will pass this portion of the exam with ease, sometimes there are unexpected results that come back. The purpose of this blog post is to explain in detail what common uterus abnormalities are, and happens afterwards in order to move forward in the surrogacy journey.

Luckily most of these conditions can be corrected with a minor procedure and the surrogate can move forward with IVF. In some rare cases, unfortunately the surrogate will not be able to carry a baby for intended parents.

Embryo Transfer and the Uterus

An embryo transfer procedure refers to the transfer of an embryo(s) from a culture dish in an IVF lab to the uterus of an intended parent or surrogate. On average, each embryo transfer has around a 50-60% live birth rate (meaning that it will result in a healthy live birth), though many factors can affect this rate. In general, an embryo’s components (e.g. its appearance and genetic makeup) and the uterine environment are the two primary factors to consider when calculating the success of an embryo transfer. This guide will explain how the uterine environment can affect the outcome of an embryo transfer.

All About the Uterus

The uterus is a hollow, muscular organ that resides in the lower abdomen (pelvis) between the ovaries, in front of the rectum, and behind the bladder. The Fallopian tubes connect to the uterus on its right and left sides, and the cervix connects the bottom of the uterus to the vagina. The uterus consists of three layers; embryo implantation and fetal development occur in the innermost layer, known as the endometrium.

The endometrium is not always suitable for embryo implantation. At the start of each menstrual cycle, the uterus sheds its endometrial lining (this is known as a period) if embryo implantation does not occur. As a result, the endometrium becomes thin and is no longer a suitable environment for embryo implantation. However, as the menstrual cycle progresses, the endometrium begins to thicken with nutrients and blood as the hormones estradiol (E2) and progesterone (P4) are produced by cells in the ovaries. By the time an embryo reaches the uterus, the endometrium should be thick and vascular, which provides the optimal environment for embryo implantation and fetal development. If implantation does not occur, the menstrual cycle will begin again.

Embryo Transfer Cycles

During a medicated embryo transfer, the hormone E2 is often administered daily at the start of the menstrual cycle. Near the middle of the cycle (when ovulation would normally occur), the hormone P4 is often administered daily. The embryo transfer is usually scheduled 3-5 days after the start of P4 administration (depending on which day the embryo was frozen on). Administering these hormones causes the endometrium to thicken as it would during a normal menstrual cycle to provide an optimal environment for embryo implantation. Some embryo transfer cycles are unmedicated, or natural, and rely on the body’s own hormones to thicken the endometrium in preparation for embryo implantation.

Regardless of which type of cycle is performed, the endometrium should be measured prior to the embryo transfer to ensure that it is thick enough (usually at least 8mm) for embryo implantation to occur. If the endometrium is not developing properly, the embryo transfer may need to be canceled.

Uterine Physical Abnormalities Can Affect Embryo Implantation

Diagnostic procedures such as a hysterosalpingogram (HSG), saline sonohysterogram (SHG), hysteroscopy, and laparoscopy can detect certain uterine abnormalities that can interfere with embryo implantation and fetal development, such as if the:

  • Uterus is abnormally shaped
  • Uterine cavity is not hollow
  • Fallopian tubes are blocked or filled with fluid (hydrosalpinx), which can leak into the uterus
  • Endometrium contains polyps or fibroids (abnormal growths), scarring (adhesions), or a septum

In many situations, these abnormalities can be corrected or removed to improve the chance of embryo implantation. In other cases, the abnormalities may not be harmful enough to require removal or treatment. And, in some cases, the abnormalities cannot be treated or removed and an embryo transfer may not be recommended.

Uterine Bacteria Can Affect Implantation

In some cases, there are abnormalities in the uterine environment that cannot be detected even with the above procedures. Many of these abnormalities require advanced testing, such as an endometrial biopsy, in order to be detected. However, these abnormalities are not very common and the following tests are not required for each embryo transfer. Talk with your doctor for more information about these tests.

The presence of bacteria in the uterine environment can affect embryo implantation. While some bacteria, such as lactobacillus, aid in embryo implantation and uterine function, other bacteria are harmful and can negatively impact embryo implantation. If there is not enough good bacteria and/or too much bad bacteria in the uterus, it can affect embryo implantation.

In some cases, the prolonged presence of bad bacteria in the uterus (known as chronic endometritis) can also have negative effects on your fertility. It is estimated that chronic endometritis affects up to 30% of infertile patients. All of these abnormalities can be detected through endometrial biopsies. Treatment options for these abnormalities often include the use of antibiotics and/or probiotics.

Endometrial Receptivity May Impact Implantation

In some situations, the endometrium is not at its most receptive point for embryo implantation on the day of the embryo transfer. A diagnostic test known as the endometrial receptivity assay (ERA) was developed to determine if an embryo transfer date should be altered so that the embryo is transferred when the uterus is most receptive for embryo implantation. The ERA requires an endometrial biopsy sample, which is taken during a menstrual cycle prior to the embryo transfer cycle. Therefore, the ERA may delay the timing of your embryo transfer.

Systemic Conditions Can Affect Implantation

Not surprisingly, certain systemic (throughout the body) conditions can also impact embryo implantation in the uterus. Examples of these conditions are autoimmune, blood clotting, or thyroid disorders. These conditions are not common but are worth mentioning since they can affect embryo implantation even though they do not originate in the uterus. Keep in mind that these conditions are rare and you do not need to test for these conditions prior to an embryo transfer.

Putting it all Together

The uterus plays a major role in embryo implantation and fetal development. In fact, the uterine environment is one major factor to consider when calculating the success rate of an embryo transfer. Ideally, a uterus should have a:

  • Normal overall shape and appearance
  • Hollow cavity
  • Clear endometrium (free of obstructions or fluid from the Fallopian tubes)
  • High level of good bacteria and low (or absent) level of bad bacteria
  • Normal window of receptivity
  • An absence of inflammation or other effects caused by systemic conditions such as thyroid disorders

These uterine abnormalities have the ability to impact embryo implantation and fetal development, but there are fortunately many tests available to detect these abnormalities. And, once detected, many of these abnormalities can be treated to improve the success rate of an embryo transfer.

It is important to know how these abnormalities can affect embryo implantation and fetal development. In many cases, it’s believed that the embryo is the cause of a failed embryo transfer, while the actual cause could potentially be a uterine condition. This guide provides some possible causes of failed implantation or miscarriage that can be further evaluated if you continue to have failed embryo transfers with good quality, genetically normal embryos. Ask your doctor if you have questions about the available methods for detecting uterine abnormalities that can affect embryo implantation or fetal development.

What Happens After the Embryo Transfer

After the embryo transfer in the in IVF process, several key steps and considerations come into play. Your fertility clinic will provide specific guidelines on activity restrictions. While some clinics recommend strict bed rest for a day or two, others may advise you to resume your normal activities gradually. The idea behind bed rest is to reduce stress on the uterus and promote implantation.

Your fertility clinic will schedule follow-up appointments to monitor your hormone levels and assess the progress of the embryo’s development. This may involve blood tests and ultrasounds to check for signs of pregnancy.

Approximately 10-14 days after the embryo transfer, you will undergo a blood test to measure the level of the hormone human chorionic gonadotropin (hCG) in your system. Elevated hCG levels indicate a successful pregnancy. If the test is positive, you will continue with prenatal care. If it’s negative, your fertility specialist will discuss the next steps, which may include another IVF cycle.


Embryo Implantation Failure: Causes, Symptoms & Treatments (babygest.com)

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