amnios

(noun) thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals). Synonyms: amnion, amniotic sac.

(noun) (pregnancy) extraction by centesis of amniotic fluid from a pregnant woman (after the 15th week of pregnancy) to aid in the diagnosis of fetal abnormalities. Synonyms: amniocentesis, amnio.