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A Step By Step Guide To The IVF Process: Step Two -- Follicle Growth And Egg Maturation

Dear Readers:

This is the third 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 be posted over the next few weeks in installments.


Under the influence of FSH (follicle stimulating hormone), dormant follicles within the ovary start to grow. Measurement of the dormant follicle number is called the “antral follicle count (AFC)” and also measured by the “Anti-Mullerian Hormone (AMH)”. Both these measurements are used to give one an idea of the ovarian capacity to be stimulated, also known as ovarian reserve, similar to the FSH level. They are additional indirect measurements. Many physicians and patients believe that these two measurements actually tell them how many eggs are left within the ovaries, but that is too poad an interpretation. We do not have the technology to know how many eggs are present without doing a careful dissection of the ovaries. So these are indirect measurements that serve to give warning about your fertility. Their only use is to help predict, as much as possible, whether the ovaries will yield many follicles upon the hyperstimulation that occurs with giving increased amounts of FSH.

So the real interpretation of a low AFC or AMH is that there might be a lower number of follicles produced, and consequently less eggs retrieved. As explained previously, these are additional measurements of “ovarian reserve.” They only predict success from a statistical point of view because part of how IVF enhances your chances of fertility is by increasing the number of eggs that are available for fertilization and hence the number of empyos and hence the increased chance of finding the perfect empyo that will lead to a pregnancy as explained in the previous segment. It is a total misunderstanding or misinterpretation to say that a low AFC or low AMH indicates that you are infertile, that your ovaries won’t stimulate or that you won’t have good eggs! Taken together with an elevated FSH, these measurements serve as red flags from a time point of view. It means that you may not have as much time to get pregnant using your own eggs as you might have thought. Since we cannot predict when you will run out of time, time becomes a critical consideration.

Currently, transvaginal ultrasound is used to monitor the follicular growth by simply measuring the follicles. This measurement is usually an average diameter taken from a horizontal and vertical measurement of the follicle and reported in millimeters (mms). As the ovary is stimulated with FSH, some of the follicles will grow. Follicles grow approximately 2 mms per day so there is some predictability of when the follicles will reach the appropriate size for ovulation or retrieval. As the follicle expands, Estradiol hormone is produced in increasing amounts by the growing follicle and so estradiol levels can be monitored to also help determine progress as well. With IVF, the goal is to have 15-20 total follicles and estradiol levels between 2000-4000. Each mature follicle will produce approximately 150-250 of estradiol. In IVF, we want to keep the estradiol level at less than 4000 because if there are more than 20 growing follicles and the estradiol level goes above 4000, there is an increased risk for an illness called “ovarian hyperstimulation syndrome”. That is a whole other topic so it won’t be explained here. Suffice it to say that OHSS has the potential to cause death in its worst form. A competent physician with experience doing IVF will take appropriate precautions to prevent this from occurring.

It is known that the follicle has to reach an average diameter of a minimum of 15 mms for the egg within to be mature. We cannot see the egg because it is microscopic size. Therefore, maturation is assumed by the size of the follicle, as has been shown in early IVF studies. With most IVF clinics, a follicle is deemed to be mature size and appropriate to trigger once it has reached at least 18 mms, but it can be as low as 15 mms based on previous studies. Because the follicles will grow unevenly, meaning there will be some that grow faster and some that grow slower, most physicians will trigger with HCG when the largest 2-4 follicles reach maturity size, or when the highest number are between 15-24 mms. My preference is for the larger follicles to be 20-24 mms which I have decided to use based on my long term experience. I don’t necessarily trigger when the largest ones reach that size but, rather, I want to get as many follicles into the mature stage as I can without losing the larger ones or have too many smaller ones. The problem with smaller sized follicles is the eggs within them will not have had adequate time to mature and so will be unusable. Also, follicles that grow to over 24 mms tend to have eggs that are over-mature and therefore not viable. Once the majority of the follicles reach a size of 20-24 mms, then you are ready for the “trigger” shot. The decision of when to give this shot is determined by the experience of the doctor part of the art of IVF. If given too soon, you may lose eggs because they will not be mature. Too late and you may lose them because they will be over-mature. The goal is to try to get the majority number of mature eggs as possible because only mature eggs will fertilize.

Until the trigger shot is given (or the body goes through an LH surge if allowed to occur naturally) the egg within the follicle does not go through its final phase of maturation, meiosis stage 2. Eggs within the follicle are usually in the “germinal vesicle (GV)” stage. Once stimulation occurs, they then go through meiosis phase 1 (M1) and then are mature at meiosis phase 2 (M2). In the natural reproductive process, the “trigger” occurs under the influence of a hormone called LH (luteinizing hormone) and is known as the LH surge. This is what is being checked when you use an ovulation detector kit. There is a sudden rise in the LH hormone which then signals the ovary to begin the ovulation event.

In IVF, HCG (human chorionic gonadotropin) hormone, which is chemically similar to LH, is substituted for the LH to make the eggs go through their final maturation phase and begin the process of ovulation. There are three sources for this medication:

(1) Urinary HCG extracted from human urine,

(2) Recombitant HCG (synthesized HCG) and

(3) Lupron, another drug that has a similar chemical structure to LH.

Lupron can only be used if you are on an antagonist protocol, with Ganerelix or Cetrotide, and not in a long Lupron protocol. This trigger shot will also cause the ovary to begin the ovulation process but because we don’t want the ovulation to occur, and thereby lose the eggs into the pelvis, the egg retrieval procedure is timed to occur before ovulation will take place. This is usually scheduled for 35-36 hours from the trigger shot.

We will continue this discussion soon with the next installment, "Step Three And Four: Egg Retrieval". 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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