The eggs that are harvested during egg extraction can be fertilized in a test tube with sperm from your partner or a donor. This is called in vitro fertilization (IVF). It's done in the laboratory, not in your body. The fertilized eggs grow briefly into tiny embryos and are then frozen.
After breast cancer treatment, if you're ready to get pregnant but your periods have not returned, the embryos are implanted in your uterus. Most clinics try not to transfer more than two at one time to avoid the risk of more than a twin pregnancy. Any number of embryos — or none — might successfully implant and start to develop.