During the course of natural insemination, sperm compete vigorously to fertilize a single egg. In the human body, this means each sperm swims through difficult conditions in the vagina, cervix, uterus and fallopian tubes, at a relative distance equivalent to a human swimming from Los Angeles to Australia. Less than 1% of the sperm ultimately arrive at the egg’s perimeter, and few are able to pierce its outer core.
During an IVF cycle, many clinics will fertilize the woman’s eggs using conventional fertilization (CI). In this case, an embryologist uses all of the eggs retrieved from the woman’s ovaries and surrounds each in a petri dish with carefully-prepared droplets of sperm to facilitate fertilization. CI requires skill and if you elect to do it, you will want to go to a clinic that does it routinely. The major fear with CI is that in roughly 0 - 15% of cases, none (or very few) of the eggs fertilize. This is called Total Fertilization Failure (TFF) and is a devastating result for patients.
During IVF, when sperm cannot fertilize an egg using CI, doctors will recommend selecting a single sperm that looks promising (the methods here are incredibly crude) from a semen sample and injecting it directly into the egg. ICSI is microsurgery, and it’s the smallest procedure we do in medicine – by injecting one cell into another, we run the risk of doing real damage to the machinery of the egg, especially if the procedure is not done carefully.
Unlike with conventional insemination (CI), ICSI can only be performed on “mature eggs” which often means we’re excluding 20 - 30% of the eggs we retrieve. Of the remaining eggs, a high quality laboratory should be able to fertilize 70 - 80% (known as the “ICSI Fertilization Rate”). What happens to those mature eggs we cannot fertilize? Often about half of those are damaged in the ICSI process.