amniotic sac       
noun [ C ]  medical specialized uk/ˌæm.ni.ɒt.ɪk ˌsæk/ us/ˌæm.ni.ɑː.t̬ɪk ˌsæk/
 羊膜囊,胎囊,羊水袋
a bag of liquid that surrounds the embryo of a mammal, bird, or reptile inside its mother
  In the centre of the amniotic sac, a little heart was beating away. 在羊膜囊的中心,跳动着一颗小小的心脏。 
 An amniotic sac contains a fetus during pregnancy.  怀孕期间,胎儿包裹在羊膜囊里。 
 Synonym: amnion
 - More examples
 - During pregnancy, the amniotic sac is formed within the uterus and encloses the fetus.
 - Part of the membrane around the baby's amniotic sac burst.
 - The amniotic sac bursts during the birthing process.