Plural of Fetus

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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.

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The Standard Rules for Forming the Plurals

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.)
Another Interactive Exercise
Here are three randomly selected questions from another exercise on plurals, which can be edited, printed to create an exercise worksheet, or sent via email to friends or students.

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See Also

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