A microscopic glass tube containing sperm (injection pipet) is used to penetrate and deposit one sperm into the egg. After culturing in the laboratory overnight, eggs are checked for evidence of fertilization. After incubation, the eggs that have been successfully fertilized (zygotes) or have had 3 to 5 days to further develop (zygotes or blastocysts) are selected. Two to four are placed in the uterus using a thin flexible tube (catheter) that is inserted through the cervix. The remaining embryos may be frozen (cryopreserved) for future attempts.