amnion

noun

am·ni·on ˈam-nē-ˌän

-ən

plural amnions or amnia ˈam-nē-ə

: the thin inner membrane of the amniotic sac that fills with the amniotic fluid and that in placental mammals also contains the umbilical cord

: a membrane analogous to the amnion and occurring in various invertebrates

amniotic

ˌam-nē-ˈä-tik

adjective

Examples of amnion in a Sentence

Recent Examples on the Web

Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.

The amnion, or innermost layer of the placenta, is filled with amniotic fluid that offers cushion and protection. — Beth Warren, The Tennessean, 2 July 2025 The amnion is the innermost layer of the placenta that surrounds the fetus. — Beth Warren, The Tennessean, 2 July 2025 Several important features of real embryos are present in the mimics, the team says, including the predecessors of sperm and eggs and the beginnings of the amnion, the membrane-like structure that encloses the embryo in a watery environment. — Bymitch Leslie, science.org, 17 June 2023 Birds, mammals and reptiles called amniotes produce eggs that include an amnion, or inner membrane, that keeps the embryo inside from drying out. — Ashley Strickland, CNN, 17 June 2020 Additional features can be built into these sensors such as amnion fluid infusion and extraction. — IEEE Spectrum, 13 Dec. 2018 Stem cells, platelet rich plasma (PRP), placenta/amnion and their variants are the latest rage. — Profootballdoc, sandiegouniontribune.com, 1 Sep. 2017

Word History

Etymology

borrowed from early Medieval Latin, borrowed from Greek amníon, probably from amnós "lamb" + -ion, diminutive suffix — more at yean

First Known Use

1661, in the meaning defined at sense 1

Time Traveler

The first known use of amnion was in 1661

See more words from the same year

Browse Nearby Words

amniochorion

amnion

amnionic

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Cite this Entry

Style

“Amnion.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/amnion. Accessed 27 Aug. 2026.

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Kids Definition

amnion

noun

am·ni·on ˈam-nē-ˌän

: a thin membrane forming a closed sac about the embryo or fetus of a reptile, bird, or mammal and containing a watery fluid in which the embryo or fetus is immersed

Medical Definition

amnion

noun

am·ni·on ˈam-nē-ˌän, -ən

plural amnions or amnia -nē-ə

: a thin membrane forming a closed sac about the embryos and fetuses of reptiles, birds, and mammals and containing the amniotic fluid

called also amniotic sac

amnion

noun

Examples of amnion in a Sentence

Word History

Browse Nearby Words

Cite this Entry

Kids Definition

Medical Definition

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