羊 yáng literally means 'sheep', but in anatomy
羊水 yángshuǐ ('sheep water' = amniotic fluid) and
羊膜 yángmó ('sheep membrane' = amnion) are simply Chinese calques of the Greek-derived English term — the imagery comes from the soft, sheep-like sac. Once you know
羊水,
羊膜 is automatic.
