The word 'embryo' and the closely related word 'fetus' have been used since the 14th century to partition prenatal life. Very recently two new compound words have been introduced to denote a further partition of early development. This article stresses that the definitions of these words denote arbitrary phases in a continuous process, and therefore have little precise scientific use. In rigorous ethical debate such arbitrary, terminology, particularly if it is used to assign moral values, should be avoided.