What are embryonic stem cells? Human embryonic stem cells come from an egg that has been fertilized in a lab as part of the in vitro fertilization (IVF) process for couples that struggle with infertility. When this process is completed, there are often excess embryos that will never be used for reproductive purposes and are discarded. Instead, these embryos can be donated for scientific research, which is a choice many couples make. It is important to know that embryonic stem cells cannot and do not come from eggs fertilized inside a woman's body, nor do they come from a miscarriage or abortion. Embryonic stem cells come from a microscopic ball of cells called a blastocyst — or, an embryo that is just a few days old. To understand this stage of development, some people find it helpful to realize that the ball of cells is smaller than the period at the end of this sentence. Embryonic stem cells are pluripotent, which means they are able to develop into nearly any of more than 200 types of cells in the body. Scientists create what are called "stem cell lines" or cells that are grown in culture and consistently multiply genetically identical cells for use in research to observe diseases or test treatments and cures.