To maximize chances of achieving a pregnancy, ovarian stimulation is used to produce many follicles (each containing an egg). During a normal, unstimulated cycle, only a single follicle develops and one egg is ovulated. Growth and development of follicles are monitored very carefully during a stimulated cycle. When the follicles (and eggs) are mature, a hormone known as human chorionic gonadotropin (hCG) is given and egg retrieval is scheduled 34-36 hours later.
Egg retrieval is usually performed under light anesthesia and using ultrasound guidance, but under certain circumstances, may require laparoscopy. The follicular contents are aspirated and handed to a laboratory technologist who then looks for eggs. The eggs are placed in dishes containing culture medium and incubated in a chamber maintained at body temperature. Shortly after the eggs have been retrieved, the male partner produces a semen sample. The semen is processed to isolate the most vigorous sperm to inseminate the eggs.
The day after retrieval and insemination, the eggs are examined for fertilization. Normal fertilization is indicated by the presence of two pronuclei. One pronucleus contains the genetic material from the mother and the other from the father. The fertilized eggs are examined every day until embryo transfer. Embryos can be transferred 2 to 6 days after egg retrieval and may be from the 2- to 4-cell (2-3 days after retrieval) stages of development to blastocyst (5-6 days after retrieval).
Embryo transfer is usually a simple procedure that does not require anesthesia. Embryos are loaded into a catheter (a thin plastic tube) and catheter is gently passed through the cervix into the uterus. Ultrasound is commonly used to confirm that the catheter tip is in the proper position in the uterine cavity. Embryos that have not been transferred can be frozen and stored for use in the future.