Preimplantation embryo


In human embryonic development, a preimplantation embryo is an embryonic stage before it becomes implanted in the uterus. The term pre-embryo previously put forward as an alternative has fallen out of use.

Preimplantation in human embryonic development

The word pre-embryo is sometimes used in ethical contexts to refer to a human embryo at least between fertilization and implantation, though this term has not been adopted by the scientific community. The stage is more recognised as the preimplantation embryo.
Implantation begins about six days after fertilization, and lasts for about a week, during which time formation of the primitive streak occurs.
Use of the term pre-embryo, in the context of human development, has drawn criticism from opponents of embryo research. From scientists who have considered this categorization invalid or unnecessary. One rationale that has been advanced for distinguishing an early fertilized human conceptus from an embryo is that there is a potential for the conceptus to split into identical twins prior to implantation, and so the conceptus cannot be regarded before implantation as a single human being. However, the conceptus before implantation exhibits self-actuated activity, which has led to the assertion that it is an embryo.

Ontological status

The illustration shows the ontological status of the preembryo, embryo, and fetus:
The colors represent the maternal and paternal genetic contributions.