The earliest moments of human development—between the time of fertilization and when the embryo implants in the womb—have remained opaque to scientists. Cells in the earliest stages of human development have the ability to generate all tissues in the body and the placenta. Unfortunately, issues such as miscarriage and birth defects can occur, but are not well understood. Until recently, peering into the inner workings of the earliest stages of development has relied on the use of human embryos. But research using human embryos, typically from IVF embryos that fail to meet quality standards, is highly restricted due to ethical concerns.
