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The Ins and Outs of In Vitro Fertilization (IVF)

Embryo Transfer

Approximately three to six days after egg retrieval, the embryos will be transferred into the uterus. In some cases, your healthcare provider may recommend a process called assisted hatching (AH). This involves making a hole into the sac (zona pellucida) that surrounds the developing embryo. This is done by dissolving part of the zona coating with an acid solution or cutting it with a needle or laser. Assisted hatching is usually done on embryos that are three days old, which consist of six to eight cells.
If your embryo transfer is to include day-3 embryos, the embryos will still have to develop to the blastocyst stage (consisting of about 100 cells) before implantation can occur. In some cases, your healthcare provider may recommend waiting to do the embryo transfer until the embryos reach the blastocyst stage in the laboratory. It can take several days (usually around five or six days after fertilization) for this to occur.
Immediately before implantation, the blastocyst must "hatch" from the zona coating, which was the original outer layer of the egg. For those who use the assisted hatching process in the laboratory, it can help make it easier for the blastocyst to hatch from the zona. Although trained and experienced technicians will perform the AH process, there is still a minimal risk of damage to the embryos from the procedure.
The number of embryos chosen to transfer will depend on a number of factors, including the mother's age, how many days into development the embryo has reached (i.e., a day-3 or day-5 embryo), and certain other conditions.
As embryos develop, the embryologists will closely evaluate them and assign each a grade. However, it's important to note that there is no standard classification system used by all fertility centers. Each center creates their own system for grading day-2 and day-3 embryos; many centers use the Gardner grading system for blastocysts.
One of the most common questions that may come up at this point is, "What does the grade of my embryo tell me about my chance to become pregnant?" Unfortunately, there is no clear answer to this. In some cases, embryos with poor grades can still result in a successful pregnancy, and there are some embryos that may have perfect grades that do not result in pregnancy.
Regardless of the grading system used, these evaluations cannot look at what is genetically going on inside the embryo. The basic reason for the grading system is to help determine which embryos to transfer and which ones to freeze.
Because embryos reach certain development criteria each day, there are different grading systems for a day-3 embryo (cleavage stage) versus a day-5 embryo (blastocyst stage). Cleavage-stage embryos are graded based on two criteria:
  • The number of cells in the embryo
  • Appearance of the embryos using a high-power microscope.
Embryos may also receive a fragmentation score. Fragmentation can occur during cell division, when little pieces break off. Although a small amount of fragmentation is normal, a higher degree of fragmentation can increase the odds of an unsuccessful implantation. Also, high degrees of fragmentation have also been linked to chromosomally abnormal embryos. Although pregnancy can still be achieved with a fragmented embryo, the odds of a successful pregnancy decrease as the degree of fragmentation increases.
In general, a good, normally growing, day-3 embryo will have between 6 and 10 cells. Research has shown that embryos with these numbers of cells are more likely to develop into viable blastocysts than embryos that have fewer cells.
Determining an embryo's appearance is subjective and basically involves how the cells in the embryo look. A score is given from of 1 (being the best) to 4 (being the worst). A grade-1 embryo will typically have cells that are all the same size and there is no fragmentation. For a grade-4 embryo, the cells may be unequal and fragmentation may be heavy.
Evaluating blastocysts is more complex. By day 5, an embryo not only continues to have cell division, but the cells are also growing and differentiating into specific types. They are also starting to outgrow the space inside the zona pellucida (outer layer of the embryo). Each blastocyst should contain two cell types:
  • One cell type that forms the inner cell mass (ICM), which eventually grows into the fetus
  • One cell type that is the trophectoderm epithelium (TE), which will make vital tissues needed to support the pregnancy (such as the placenta).
When grading blastocysts, a letter grade is assigned to each of the cell types. In some cases, a grade may also be assigned to the fluid-filled cavity (blastocoel). With this system, the ranking goes from A-quality (the best) to D-quality (the worst). The embryos may also be graded on how much they have expanded, including very early blastocyst, expanded blastocyst, and hatched blastocyst.
When you go in for your embryo transfer, your healthcare provider will go over how many embryos you have, the grade of each, and which embryos should be transferred and how many. Determining whether an embryo has a good chance of becoming a successful pregnancy will be made by taking all of the components of the embryo into account.
One of the main risks associated with an IVF procedure is that of twins or multiples -- the more embryos that are transferred, the greater the risk. Multiple pregnancies can carry potentially significant risks, such as:
In an effort to reduce the number of higher-order multiple pregnancies, the American Society for Reproductive Medicine issued some guidelines for the number of embryos that should be transferred. These guidelines are based on:
  • The quality of the embryo
  • The stage of the embryo (cleavage-stage is two or three days past fertilization, and blastocysts are five or six days past fertilization)
  • The age of the woman at the time of fertilization.
In general, your healthcare provider should transfer the minimum number of embryos that will still provide a high chance of getting pregnant with a low risk of multiples. If you do become pregnant with more than one baby, you may want to consider having a consultation with a healthcare provider who is experienced in high-risk pregnancies. He or she can give you information and guidance about complications that may occur during your pregnancy. Some women may also want to consider reducing the number of embryos they are carrying.
The embryo transfer will typically take place three to five days after the egg-retrieval process. This is done by passing a catheter through the cervix into the uterus. An ultrasound is used as well so your healthcare provider can see the proper placement to deposit the embryos. The embryos are then deposited into the uterine cavity along with a small amount of fluid.
This transfer is usually a quick process and no anesthesia is required. Your healthcare provider may have you remain lying down for a few minutes after the procedure. You will also receive instructions for the following two weeks until it's time for the pregnancy test.
Although they can vary, your instructions for the next few days after the transfer may include lots of couch time. In many cases, you'll be asked to spend the first 24 hours after the embryo transfer with your feet up and resting as much as possible. After the first 24 hours, women are usually encouraged to take it easy for another three or four days. Although you can return to work and go back to a fairly normal routine, you should avoid strenuous exercise, chores, and sexual intercourse -- anything that may cause uterine contractions and could interfere with the implantation process.
Last reviewed by: Arthur Schoenstadt, MD
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