A designer baby is a baby whose genetic makeup has been selected or altered, often to exclude a particular gene or to remove genes associated with disease. This process usually involves analysing a wide range of human embryos to identify genes associated with particular diseases and characteristics, and selecting embryos that have the desired genetic makeup; a process known as preimplantation genetic diagnosis. Screening for single genes is commonly practiced, and polygenic screening is offered by a few companies. Other methods by which a baby's genetic information can be altered involve directly editing the genome before birth, which is not routinely performed and only one instance of this is known to have occurred as of 2019, where Chinese twins Lulu and Nana were edited as embryos, causing widespread criticism.

Genetically altered embryos can be achieved by introducing the desired genetic material into the embryo itself, or into the sperm and/or egg cells of the parents; either by delivering the desired genes directly into the cell or using gene-editing technology. This process is known as germline engineering and performing this on embryos that will be brought to term is typically prohibited by law. Editing embryos in this manner means that the genetic changes can be carried down to future generations, and since the technology concerns editing the genes of an unborn baby, it is considered controversial and is subject to ethical debate. While some scientists condone the use of this technology to treat disease, concerns have been raised that this could be translated into using the technology for cosmetic purposes and enhancement of human traits.