<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> The amniotic sac, commonly called the bag of waters, sometimes the membranes, is the sac in which the fetus develops in amniotes . It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth . The inner of these fetal membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the fetus . The outer membrane, the chorion, contains the amnion and is part of the placenta . On the outer side, the amniotic sac is connected to the yolk sac, the allantois and, via the umbilical cord, to the placenta . </P> <P> Amniocentesis is a medical procedure where fluid from the sac is sampled to be used in prenatal diagnosis of chromosomal abnormalities and fetal infections . </P> <P> The amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid . The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo . As the amniotic fold rises and fuses over the dorsal aspect of the embryo, the amniotic cavity is formed . </P>

When does the amniotic sac attached to the uterus