Human Embryogenesis, Fertilization And Fetus Development

Fœtus or foetus is the British, Irish and Commonwealth spelling, which has been in use since at least 1594. It arose as a hypercorrection based on an incorrect etymology (i.e. due to insufficient knowledge of Latin) that may have originated with an error by Saint Isidore of Seville, in AD 620. This spelling is the most common in most Commonwealth nations (except in medical literature, where its use is barred). The etymologically correct original spelling, fetus is used in Canada and the United States. In addition, fetus is now the standard English spelling throughout the world in medical journals. The spelling "faetus" was used historically.
The spelling in the Oxford Encyclopedic English Dictionary, Third Edition (1996), is 'foetus' with 'foetuses' as the plural; 'fetus' is given as the 'US variant of foetus.' However, later editions of the OED clarify the etymology behind the Commonwealth spelling.