Any of the following events could prevent pregnancy:
- The time of ovulation may be misjudged or may be unpredictable, or may not occur at all monitored cycle(s), thus precluding any attempt at obtaining eggs.
- An attempt at egg collection may be unsuccessful
- The egg(s), if obtained, may not be normal.
- Loss or damage of the embryos may occur during the procedure
- Fertilization may not occur or the embryo(s) may not develop normally.
- There may be other reasons which are not understood by the scientists.