The Story of Attrition

Have you ever come across the word attrition during one of your fertility appointments? If not, you probably will, as it describes one of the most fundamental principles of human reproduction.

Attrition simply means the gradual reduction in numbers over time.

The remarkable thing is that attrition shapes human reproduction long before we reach puberty. It begins before birth and continues throughout every stage of reproductive life.

Femal reproductive cells (eggs) first appear when we are tiny embryos ourselves, at around 5–6 weeks of fetal development. Their numbers then increase rapidly, reaching a peak at approximately 16–20 weeks of pregnancy. From that moment on, the direction changes. More cells are lost than preserved.

A baby girl is born with approximately one million primordial follicles. A primordial follicle is the earliest and smallest stage of a follicle in the ovary. It consists of an immature egg (oocyte) surrounded by a single layer of flat supporting cells. By puberty, only about 300,000 of them remain. By the late thirties, the number has fallen dramatically, and by menopause, only a tiny fraction (few hundreds up to one thousand) is left.

More than 99.9% of ovarian follicles will never ovulate. This lifelong decline is known as ovarian follicle attrition.

Then, years later, many women encounter the same principle again during IVF.

Perhaps ten eggs are collected. Eight are mature. Seven fertilize. Five continue dividing. Two become blastocysts. One may be chromosomally normal. Patients often describe these numbers as a series of losses.

"We started with ten eggs... how did we end up with only two embryos?"

From a biological perspective, however, this is not a story of failure. It is another example of attrition. Both processes tell exactly the same story.

Not every follicle is destined to produce an ovulatory oocyte. Not every oocyte has the developmental potential to become an embryo. Not every embryo has the capacity to reach the blastocyst stage or ultimately result in a pregnancy.

In reproductive medicine, attrition is not synonymous with failure. It is the visible expression of the natural biological selection that accompanies human reproduction at every stage. Human reproduction is remarkably selective.

Perhaps this also explains why humans are surprisingly poor at reproduction compared with many other species. We produce relatively few eggs, ovulate only one oocyte in most cycles, experience high rates of embryo loss before implantation, and have one of the highest rates of chromosomal abnormalities among mammals.

Seen this way, IVF does not create attrition. It simply allows us to witness it.

For the first time, we can observe what has always happened inside the human body but remained invisible. We see which eggs mature, which fertilize, which embryos stop developing, and which continue.

Understanding attrition does not take away the disappointment of an unsuccessful cycle. But it can take away the burden of feeling that something must be wrong with me.

One unsuccessful stimulation, one embryo that stopped developing, or one failed transfer does not necessarily mean that the treatment was wrong or that your body has failed. Sometimes, it simply means that biology was doing what biology has always done: selecting. This is why fertility specialists so often recommend trying again. Eevery new cycle gives us another opportunity to find the egg with the potential to become a healthy embryo. Understanding attrition helps explain why "let's try again" in many situations, is simply the most evidence-based decision.