The most common form of assisted reproductive technology (ART) is in-vitro fertilization-embryo transfer (IVF-ET). IVF-ET involves fertilization of eggs with sperm in vitro (outside of the body), in the laboratory. The process involves stimulating a woman's ovaries with injectable hormones (gonadotropins) to induce multiple eggs to grow, typically over a 7-9 day period. Some IVF protocols will require a "downregulation" step prior to stimulation, during which the ovaries are briefly suppressed before follicles are stimulated to grow; while in other protocols, no prestimulation suppression is needed. The prestimulation phase (with or without downregulation) may or may not involve a brief pretreatment with birth control pills. Once follicles reach a mature size (gauged by sequential vaginal ultrasound examinations and blood work obtained every 2-4 days during the stimulation phase), eggs within the follicles are triggered to mature, and then harvested in a minor surgical procedure under "twilight sleep" anesthesia. The egg retrieval procedure is done under direct transvaginal ultrasound visualization by advancing a needle through the vaginal wall and directly into the ovaries, where one follicle is aspirated at a time. Follicular fluid is examined carefully by the embryologist under microscopic visualization in an incubator for the presence of eggs. Next, eggs are mixed with sperm (typically provided by the patient's partner through ejaculation; or by thawing frozen sperm as in the case of sperm donation) or injected with sperm in a process called intracytoplasmic sperm injection (ICSI) (as in the case of severe male factor infertility), in order to achieve fertilization. Fertilized eggs are then cultured under a strictly controlled environment within specialized incubators in the IVF laboratory for 3-5 days while they develop as embryos. Finally, embryos (or an embryo) are transferred into the uterine cavity for implantation.