Plural of Fetus

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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

Read more about English spelling rules.

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.)

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