During the controlled ovarian stimulation hormonal preparations (injections) are used to stimulate the ovaries so as to produce an adequate number of follicles which contain the eggs (oocytes) that are going to be used for the formation of the embryos. COS duration is about 12-14 days until there are follicles of adequate size. This depends on the response of the ovaries which is assessed with bloods and ultrasound scans. When there are adequate follicles formed the triggered injection is administered.