Plural of Fetus

The Quick Answer
The plural of fetus is fetuses or feti .

The Plural of Fetus

The plural of fetus is fetuses or, less commonly, feti.
  • Female fetuses tend to weigh less than males, at full term.
  • The detection of multiple feti can be made by using ultrasound technology.
The noun fetus has a Latin root, which is the derivation of the plural feti. Fetuses (which adheres to the standard rules for forming plurals) is by far the more common version.

The table below shows the standard rules for forming the plurals of nouns in English.

Type Example of Type Forming the Plural Plural
Most Nouns lamp
scythe
add s lamps
scythes
Noun Ending s, sh, ch, x or z fetus
dress
add es fetuses
dresses
Nouns ending [consonant] o hero
zero
tomato
add either s or es
(There are no rules for this - you have to know.)
heroes
zeros
tomatoes
Nouns ending [vowel] o patio
ratio
add s patios
ratios
Nouns ending [consonant] y story
penny
change the y to an i and add es stories
pennies
Nouns ending [vowel] y donkey
chimney
add s donkeys
chimneys
Nouns ending f or fe dwarf
hoof
ves and/or s
(There are no rules - you have to know.)
dwarfs
hooves or hoofs
Exceptions man
louse
some nouns undergo a vowel or letters change men
lice
More exceptions salmon
sheep
some nouns do not change at all salmon
sheep
Foreign rulings fetus
medium
some nouns adopt foreign rulings feti
media

Why Is There Confusion over the Plural of Fetus?

Confusion arises over the plural of fetus because its original plural form (feti) derives from Latin and native English speakers are drawn to fetuses, which adheres to the standard ruling for forming plurals.

Both fetuses and feti are acceptable. Of note, fetuses is by far the more common plural. (See the evidence in Google's Ngram Viewer.)

See Also

Unusual pluralsPlural forming table Quirks with forming plurals Forming the plurals of abbreviations Forming the plurals of compound nouns