| a fluid-filled membranous sac in the pregnant uterus that encloses and cushions the fetus, normally breaking at or just before the time of birth; the amnion. |

| bag of waters n. See water bag. |
| water bag n. The membranous sac filled with amniotic fluid that protects a fetus during pregnancy. Also called bag of waters. |
bag of waters n.
the amniotic sac and the amniotic fluid in it