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

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. A conceptus between fertilization and implantation is also frequently classified as a pre-implantation embryo.

Even after implantation begins, a pre-embryo may exist up until the formation of the primitive streak. 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 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.

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. However, Ann Kiessling (a leader in stem cell research) has written that those categories are inaccurate.

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

The colors represent the maternal and paternal genetic contributions.