Amnion is the tough ﬁbrous membrane which lines the cavity of the womb (UTERUS) during pregnancy, and contains between 0·5 and 1 litre (1–2 pints) of ﬂuid in which the EMBRYO ﬂoats. It is formed from the ovum (egg) along with the embryo, and in labour the part of it at the mouth of the womb forms the ‘bag of waters’. (See PREGNANCY AND LABOUR.) When a child is ‘born with a CAUL’, the caul is a piece of amnion.
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