When these gonadotropins (FSH, HMG) are administered, they augment the natural pituitary FSH to stimulate follicle growth. The primary mechanism of fertility enhancement is through the stimulation of multiple follicles (i.e. multiple eggs). Visualizing the ovaries by ultrasound allows determination of the number and size of the follicles in each ovary. When it is determined that the follicles should contain mature eggs, an injection of HCG (human chorionic gonadotropin – the hormone of pregnancy) is given. Ovulation usually occurs approximately 34 – 42 hours after the injection.