Figure 2. Potential range of preimplantation loss among 100 unselected couples in their first-cycle attempt of pregnancy. This takes into account the cycles in which fertilization cannot take place (dark gray area, A; see also Fig. 1) and assumes two possible estimates of ova fertilization in the presence of sperm (64% and 90%, B). In C and D, the estimated numbers of fertilized ova become the new denominators for estimating preimplantation loss. For instance, in C, there are 55 cycles with fertilized ova (64% of 86 cycles with viable ova). Given that 36 cycles achieve implantation, the number of cycles with preimplantation loss is 19 (55 minus 36), with a preimplantation loss rate of 19/55 or 35%. Pregs: pregnancies.