Why Did So Few of My Eggs Fertilize and Make it to Blastocyst?

One of the commonest questions asked by patients undergoing IVF relates to the likelihood of their eggs fertilizing and the likely “quality of their embryos. This is also one of the most difficult questions to answer. On the one hand many factors that profoundly influence egg quality; such as the genetic recruitment of eggs for use in an upcoming cycle, the woman’s age and her ovarian reserve, are our outside of our control. On the other hand the protocol for controlled ovarian stimulation (COS) can also profoundly influence egg/embryo development and this is indeed chosen by the treating physician.

First; it should be understood that the most important determinant of fertilization potential, embryo development and blastocyst generation, is the numerical chromosomal integrity of the egg (While sperm quality does play a role, in the absence of moderate to severe sperm dysfunction this is (moderate or severe male factor infertility a relatively small one). Human eggs have the highest rate of numerical chromosomal irregularities (aneuploidy) of all mammals. In fact only about half the eggs of women in their twenties or early thirties, have the required number of chromosomes (euploid), without which upon fertilization the cannot propagate a normal pregnancy. As the woman advances into and beyond her mid-thirties, the percentage of eggs euploid eggs declines progressively such that by the age of 40 years, only about one out of seven or eight are likely to be chromosomally normal and by the time she reaches her mid-forties less than one in ten of her eggs will be euploid.

Second; embryos that fail to develop into blastocysts are almost always aneuploid and not worthy of being transferred to the uterus because they will either not implant, will miscarry or could even result in a chromosomally abnormal baby (e.g. Down syndrome). However, it is incorrect to assume that all embryos reaching the blastocyst stage will be euploid (“competent”).  ). It is true that since many aneuploid embryos are lost during development and that those failing to survive to the blastocyst stage are far more likely to be competent than are earlier (cleaved) embryos.  What is also true is that the older the woman who produces the eggs, the less likely it is that a given blastocyst will be “competent”. As an example, a morphologically pristine blastocyst derived from the egg of a 30-year-old woman would have about a 50:50 chance of being euploid and a 30% chance of propagating a healthy, normal baby, while a microscopically comparable blastocyst-derived through fertilization of the eggs from a 40-year-old, would be about half as likely to be euploid and/or propagate a healthy baby.

While the effect of species on the potential of eggs to be euploid at ovulation is genetically preordained and nothing we do can alter this equation, there is, unfortunately, a lot we can (often unwittingly) do to worsen the situation by selecting a suboptimal protocol of controlled ovarian stimulation (COS). This, by creating an adverse intraovarian hormonal environment will often disrupt normal egg development and lead to a higher incidence of egg aneuploidy than otherwise might have occurred.  Older women, women with diminished ovarian reserve (DOR) and those with polycystic ovarian syndrome are especially vulnerable in this regard.

During the normal, ovulation cycle, ovarian hormonal changes are regulated to avoid irregularities in production and interaction that could adversely influence follicle development and egg quality. As an example, small amounts of androgens (male hormones such as testosterone), that are produced by the ovarian stroma (tissue surrounding ovarian follicles) during the pre-ovulatory phase of the cycle enhance late follicle development, estrogen production by the granulosa cells (that line the inner walls of follicles), and egg maturation. However, over-production of testosterone can adversely influence the same processes. It follows that COS protocols should be individualized and geared toward optimizing follicle growth and development time while avoiding excessive ovarian androgen (testosterone) production and that the hCG “trigger shot” should be carefully timed.

In summary, it is important to understand the influence species, age of the woman as well as the effect of the COS protocol can have on egg/embryo quality and thus on IVF outcome. The selection of an individualized protocol for ovarian stimulation is one of the most important decisions that the RE has to make and this becomes even more relevant when dealing with older women, those with DOR and women with PCOS. Such factors will in large part determine fertilization potential, the rate of blastocyst generation and indeed IVF outcome.

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