A system map of staged biological loss from eggs → embryos → transfers, and how quantity collapses into probability.
Stage-Based Loss Is Expected
Not all retrieved eggs mature. Not all mature eggs fertilize. Not all fertilized eggs reach the blastocyst stage. Not all blastocysts are chromosomally viable. This staged loss is biologically normal and unavoidable.
Age Drives Funnel Steepness
Maternal age is the dominant variable shaping attrition. As age increases, chromosomal error rates rise, steepening loss between fertilization and viable embryo selection regardless of egg count or stimulation response.
Laboratory Performance Modulates Loss
Culture environment, embryologist technique, and handling protocols influence how much attrition occurs before embryos reach selection. Laboratory quality alters where loss occurs, not whether it occurs.
Preimplantation genetic testing does not cause attrition. It exposes it. Embryos deemed non-viable would otherwise fail to implant or miscarry later in the process.
Transfers Are Not Equal Units
A transfer represents a probabilistic event, not a guaranteed outcome. Multiple transfers may be required from a single retrieval, and some retrievals may yield no transferable embryos.