Pre-embryo

In human embryonic development, a pre-embryo is a conceptus before implantation in the uterus.^[1]

The word pre-embryo is sometimes used in ethical contexts to refer to a human conceptus at least between fertilization and implantation, though this term has not been adopted by the scientific community.^[2] A conceptus between fertilization and implantation is also frequently classified as a pre-implantation embryo.^[2]

Even after implantation begins, a pre-embryo may exist up until the formation of the primitive streak.^[3] Implantation begins about six days after fertilization, and lasts for about a week, during which time formation of the primitive streak occurs.^[4]

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.^[1][5][6] 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 argument goes) 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.^[7] Further, identical twinning is an instance of asexual reproduction whereby a conceptus, without ceasing to be what it is (a new human being), provides a cell or cells as a new conceptus, entirely separated or partially separated (a 'siamese' twin) from the original conceptus, but in any event self-actuated in its development from the moment that the act of asexual reproduction (twinning) is complete.^{[citation needed]} By this asexual reproduction, the parents of the original conceptus in effect become grandparents to the identical twin so conceived.^{[citation needed]}

In the United States, a report by the National Institutes of Health stated that a conceptus could be both a pre-implantation embryo and a pre-embryo at the same time.^[8] However, Ann Kiessling (a leader in stem cell research) has written that those categories are inaccurate.^[9]

The illustration shows the ontological status of the preembryo, embryo, and fetus:

The colors represent the maternal and paternal genetic contributions.

• Beginning of pregnancy controversy
• In vitro fertilization
• Pregnancy
• Stem cell controversy