HCG is a hormone that is administered 36 hours before egg collection. In some cases, a GnRH analog is administered instead.
These hormones are responsible for facilitating the final process of oocyte maturation and ovulation. It is crucial to administer these hormones at the time indicated by your assisted reproduction specialist, as doing so too early could cause premature ovulation, therefore preventing oocytes recovery when performing the egg collection.
Administering the trigger injection too late could lead to a lack of maturation of the oocytes, which leads to a lower oocyte recovery rate (because part of these cumulus-oocyte complexes will still be adhered to the follicle wall, and therefore will not be recovered in the egg collection) and that the collected oocytes will probably be immature and not suitable thus to be used for the treatment.