Clomiphene is usually prescribed for five days each cycle, usually beginning on day three or five. Of all women treated with clomiphene 60% to 80% will ovulate normally. Injectable hormonal preparations are also used to induce ovulation in women who do not respond to oral medicines. Response to these drugs is monitored by frequent vaginal ultrasounds. At a time in the cycle when the ovarian follicles reach a designated size, an injection of the hormone HCG is given to trigger ovulation. Ovulation usually occurs 36-48 hours after the HCG injection. Thus, intercourse or insemination should be timed accordingly.