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A Step By Step Guide To The IVF Process: Step Five -- Fertilization

Dear Readers:

This is the fifth part in the series I have begun to help answer what In Vitro Fertilization (IVF) is and how it works with my world-wide Blog audience. What you read here is what I also provide my patients with on a daily basis. I plan on going into some detail but in a way that is understandable to the normal (lay) audience, and not the medical or scientific one. I also hope that this will not only clarify what you will go through, but explain why things are done a certain way and what the goals of each step are. I also want to convey that IVF is actually a replacement for some of the “natural” steps required to get pregnant and not some miraculous high tech fertility treatment that gets patients pregnant artificially, as many think it is. It is somewhat of a miracle that we can do as much as we can, but there are still lots of things/steps that we cannot do or influence. I hope this discussion will benefit you. This series will continue to be posted over the next few weeks in installments. (For earlier installments in correct order scroll down to the beginning of July 2012.)


In the previous step, the eggs were aspirated and retrieved from the ovaries, the equivalent of ovulation in the natural process. These eggs have been collected and the empyologist now isolates each egg within a petri dish. The eggs are prepared for the fertilization step. In the natural cycle, after ovulation, the egg needs to pass through the culdesac of the pelvis to find the fimpia of one the tubes. In the normal anatomical position, the fimpia of the tubes are lying within the culdesac. If there is scar tissue within the culdesac or the anatomical positions of the tubes are altered, then this “pickup” step may not occur. This would be an indication to do IVF. If this does not occur, then that is the end of the process in that cycle. If it does find the tube, it is then pought into the tube and through peristalsis (muscular motion) within the tube it is pushed to a portion of the tube where the sperm need to be waiting to accomplish fertilization. Again, if the sperm are not there, then fertilization will not take place and the cycle ends at that point. If the sperm is in the correct position at the correct time, then sperm attach to the egg and one sperm penetrates the egg to accomplish fertilization. In IVF, these two steps are accomplished by laboratory means, so there is little chance that those steps will not be accomplished. This is another step that IVF accomplishes for your body, thereby enhancing your chances of success. It is not left to chance as in the natural process.

At this point, there are two ways to accomplish fertilization. It can be allowed to occur by natural means, meaning the sperm can be added and the egg-sperm interaction allowed to happen on its own, or it can be assisted, also known as ICSI or "intracytoplasmic sperm injection". In order for the fertilization process to occur, a sperm needs to penetrate the outer lining (shell) and enter the egg and then there is an elaborate process within the egg whereby the nucleus of the egg and the nucleus of the sperm unite. This latter process is the actual process of fertilization. Just putting a sperm into the egg is NOT fertilization. The second part of the process, a natural step, has to occur on its own. In other words, just doing ICSI is not fertilizing the egg and does not guarantee fertilization. ICSI is just putting the sperm within the egg so that fertilization has the possibility of happening.

If natural fertilization is going to be done, the egg and sperm are put together and the sperm is allowed to penetrate the egg on its own. But if injection of the sperm is going to be done, then the empyologist removes the filmy lining of the egg, called the cumulus. This will also allow evaluation of the maturity of the egg. Only mature eggs, in Meiosis stage II, can be fertilized. Eggs can be in either Meiosis stage II (M2), Meiosis stage I (M1) or Germinal Vesicle (GV) stages at the time of retrieval. As explained previously, the follicles, and the eggs within, don’t all mature at the same rate. That leads to some larger follicles and some smaller follicles. The larger follicles have the matured eggs, eggs that have had the time to mature, and the smaller follicles have immature eggs. As a result, eggs in the various stages can be retrieved, but only the mature ones will fertilize. In the case of Meiosis stage I eggs, these can be left in culture and allowed to mature, if the clinic has an egg maturation program and the proper media in place, or in some cases, just waiting will allow the egg time to mature to the mature Meiosis stage II. They can then be fertilized at that time. So with ICSI, an anatomically normal appearing and swimming sperm is identified and aspirated into a glass needle. This needle is then inserted into the egg and the sperm is then injected within. All of this procedure is done microscopically. ICSI essentially assists the step whereby the sperm enters the egg. This procedure is done mostly in cases where there is an abnormality of the semen analysis, since it is known that with abnormal semen analysis, there is a high possibility of a functional problem i.e. that the sperm cannot fertilize the egg. Also, it is recommended in older patients (>37 years old) because the shell of the egg is thicker and sperm have a more difficult time penetrating the shell normally. In some cases, patients will choose to have ICSI because they don’t want to take the risk of fertilization not taking place (fertilization failure), which will essentially end the cycle at that point. Without fertilization, empyos are not formed, and no empyos means there will be nothing to transfer.

Once the eggs are put with the sperm, the petri dishes are placed into an incubator. Fertilization will take a minimum of 12 hours to occur so most clinics will re-evaluate the eggs the next morning for fertilization. It is expected that some eggs will fertilize normally, called a 2PN stage, some will not fertilize and some will fertilize abnormally (1PN or 3PN). Because of this, there will be another reduction in the number of eggs available to use. With fertilization, the egg is now referred to as an empyo. A typical threshold to evaluate fertilization is to expect a minimum of 50% of the available (mature) eggs to fertilize. If that threshold is not reached, it could be because of egg quality, sperm quality or lab quality. Only the fertilized empyos can be cultured to develop into empyos that can be transferred back into the womb, so this number essentially establishes how many might be available for transfer. Fertilized empyos are now transferred into a new culture media and placed in incubators that have a specific gas content and temperature to allow them to grow.

We will continue this discussion soon with the next installment, "Step Six: Empyo Development". Thank you for joining me today!

Edward J. Ramirez, M.D. F.A.C.O.G.

Medical Director, Monterey Bay IVF

Monterey, CA

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