Fetus
WordNet
noun
(1) An unborn or unhatched vertebrate in the later stages of development showing the main recognizable features of the mature animal
WiktionaryText
Noun
- An unborn or unhatched vertebrate showing signs of the mature animal.
- A human embryo after the 8th week of gestation.
Usage notes
Generally, is accepted as the correct spelling especially in the scientific community whereas, is still commonly used in Commonwealth countries such as the UK.