Maturation in the laboratory of the egg (as a primary oocyte) obtained from an immature tertiary follicle until it becomes a secondary oocyte competent to be fertilised by sperm using in vitro fertilisation. The smaller the follicle, the lower the proportion of eggs that mature successfully. ICSI can be used to increase the proportion of eggs that will fertilise, but the embryos on average do no better, and so no advantage is conferred by ICSI in this situation.