Schematic illustration of treatments in FRESH-MII (Control 1), VW-MII (Control 2), V/W-AFTER-IVM, and V/W-BEFORE-IVM groups.

<p>The duration of oocyte maturation was 15 hours in all groups, which used hCG trigger as a set point. In FRESH-MII group, the freshly collected IVO MII oocytes were injected with a spermatozoa without any vitrification. In VW-MII Group, IVO MII oocytes were harvested 15 h after the hCG trigger from the oviducts, and those IVO MII oocytes were vitrified for later ICSI. In V/W-AFTER-IVM Group, cumulus cells were removed from MI oocytes, which collected 6–7 h after the hCG trigger from ovaries. After another 8 h of <i>in vitro</i> maturation, the oocytes with a PB extruded (IVM MI-II oocytes) were vitrified for later ICSI. In V/W-BEFORE-IVM Group, cumulus cells removed MI oocytes, which collected 6–7 h after the hCG trigger from ovaries, were vitrified for storage. After oocyte warming, the MI oocytes were <i>in vitro</i> matured for another 8 h, and the oocytes with a PB extruded (IVM MI-II) were regarded as mature oocytes for ICSI.