CelC, it takes more than egg maturity for fertilisation to take place. Size is one indicator of maturity. Depends on the internal make-up of the egg and the sperm too to see whether they can interact and allow fertilisation to take place. Guess only the embryologists can know the details. Furthermore, if u're doing IVF (and not ICSI), it only means that they leave the eggs and sperm together in a petri dish and let fertilisation take place on its own. So if the sperm cannot penetrate the eggs, no fertilisation takes place at all. Even with ICSI, fertilisation is not guaranteed.