Chat with us, powered by LiveChat Understanding IVF ATTRITION RATE: Why all eggs don't form embryos

Understanding IVF ATTRITION RATE: Why all eggs don't form embryos

By MomMed | 29 June 2022 | 0 Comments


It can be exhilarating when you have made the decision to build your family through the process of In Vitro Fertilization, also called IVF. However, one of the concerns of many couples trying to conceive is to know what their chances of success are. Everyone would of course love to see a perfect score, possibly a 100, but this is not ultimately realistic. Before we look into the reason this is so, let's have a quick recap of the IVF process.


IVF is the process whereby a man’s sperm and a woman’s egg are combined outside of the body to form a fetus, which is thereafter transferred into the woman’s body for onward growth, maturation, development and delivery. It is a form of Assisted Reproductive Technique (ART) used to assist couples who are finding it difficult to achieve conception through natural sexual intercourse. Studies have shown that IVF is the most common and effective assisted reproductive technique.


Each step in IVF requires a technique. These various techniques are referred to as principles and they include the following:
OVARIAN STIMULATION: IVF involves controlled ovarian stimulation using medications such as clomiphene and GnRH (Gonadotrophin-releasing hormone) analogues. This results in the formation of multiple dominant ovarian follicles rather than a single one as would be expected in a normal menstrual cycle. This is done to increase the number of eggs available for retrieval. 
EGG RETRIEVAL: This is the harvesting of mature viable eggs released from the stimulated dominant ovarian follicles, usually prior to ovulation at the peak of growth. A natural menstrual cycle usually gives rise to only one dominant follicle, however in IVF, multiple follicles are stimulated into dominance, leading to the release of multiple eggs.
INVITRO FERTILIZATION: This stage involves the fertilization of the harvested eggs by sperm, usually in a petri dish and resulting in the formation of embryos.
IMPLANTATION: This involves the placement of the embryos in the uterine cavity. A woman can also choose to preserve the formed embryos by freezing them in the IVF laboratory for later implantation such that she does not have to go through the first three stages in her subsequent fertility journey. 


Attrition simply means a reduction in strength or number. In this context, IVF attrition is defined as the rate at which viable genetic materials decrease progressively once in the IVF laboratory. Attrition in IVF is a game of number and chance as it has no set pattern or predetermined format.

With respect to IVF, not all eggs retrieved eventually end up as embryos. As eggs are harvested and go through the various stages of IVF, their number tends to progressively decrease. This progressive decrease is the attrition rate of IVF. However it is comforting to note that attrition is a beneficial process of natural selection that ensures that only the healthiest and most viable of the eggs eventually get to become embryos and subsequently, babies.

There are no standard rates of attrition for IVF as it is not possible to predict the exact outcome of the process for any patient, more so as no two patients would have the same outcome.

WHAT factors increase risk of IVF attrition?

Attrition rate tends to increase with age. The reason for this is not yet fully understood, although some explanations have been proffered. It has been suggested that in addition to a possible genetic predisposition increasing risk, there is usually a decrease in ovarian reserve with age. Also, increasing age has been associated with poorer oocyte quality, lower embryo implantation rates, and altered hormonal environment, resulting in ovulatory dysfunction and uterine problems.

Also with increasing age, there is a higher propensity for acquired conditions such as endometriosis, fibroid, and pelvic infections. Similarly, age-related lifestyle changes such as obesity equally tend to increase attrition rates. 


Attrition happens at various stages of the IVF process:
EGG MATURATION: IVF process essentially begins with ovarian stimulation resulting in the development of numerous follicles that presumably contain eggs. Incidentally and for some reason, not all the stimulated follicles end up with eggs. Some of the eggs containing follicles sometimes do not grow to mature dominant follicles, these are eventually reabsorbed.
EGG RETRIEVAL POINT: It has been observed that the dominant follicles contain eggs at various stages of maturation; hence not all the eggs are harvested. Studies have shown that usually, only 80% of the eggs in the different dominant follicles are mature enough for fertilization. This is usually the first point of attrition in the IVF process.
FERTILIZATION STAGE: This is the stage where the mature viable harvested eggs are fertilized by the man’s sperm. This happens naturally in the laboratory and can be achieved via one of two means; conventional insemination or intra cytoplasmic sperm injection.


  1. Conventional insemination: Here the eggs are surrounded by a deposit of sperm in the petri dish, resulting in the fertilization of the egg by one sperm. It is a prototype of the natural fertilization process that occurs in the body after sexual intercourse. This is the method of choice in the presence of healthy sperm.

  2. Intracytoplasmic Sperm Injection (ICSI): This method involves the direct injection of the sperm into the egg for fertilization. This method is usually preferred in the circumstance of doubtful sperm quality or male factor subfertility.

It is pertinent to state that this is another point of attrition in IVF as not all eggs eventually get fertilized.


This stage involves growing the fertilized egg, now referred to as the embryo, in the laboratory over a period of 5 to 6 days. This is another significant point of attrition in the IVF process. Within this period, the process of active cell division begins and eventually at day 3, the 6-8 cell embryo stage has been achieved. Most of the embryos often get to this stage. Thereafter, the attrition process begins until the 6 day stage with an almost halving of the total number of initial embryos. This stage is the blastocyst stage. The blastocyst is that embryo that has over a hundred cells. It is significant to highlight that the attrition rate at this stage is the highest with a rate of 50-70%, unlike in the previous stages where the attrition rate hovers around 20%. 


In a nutshell, attrition is a natural process that ensures that only mature, viable embryo eventually gets implanted. This significantly reduces the risks of failed implantation, early trimester miscarriages, and incidences of chromosomal abnormalities.  The IVF processes is associated with varying attrition rates as no two women's fertilization processes are the same, making exact predictions difficult, although generally, women younger than 37 years tend to have lower attrition rates than older women. The emphasis however, is on the harvesting and fertilization of healthy viable eggs and subsequent implantation of equally healthy embryos.

In conclusion, attrition is a natural step in the process of conception that ensures that only viable embryos eventually get implanted, much like it happens in natural conception. IVF remains the most effective means of ART and even having a single healthy embryo at the end of the process offers you a chance to build the family that you desire.

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