Fetal membranes refer to the protective sacs surrounding a developing embryo in the uterus. These sacs are known by several synonyms, including amniotic sac, which is the largest, the chorion, the yolk sac, and the allantois. The amniotic sac, filled with amniotic fluid, cushions and protects the developing fetus, while the chorion plays a role in the exchange of nutrients and waste between the mother and fetus. The yolk sac plays a crucial role in providing key nutrients and blood cells to the developing embryo in the early stages of pregnancy, and the allantois helps with waste removal. Knowing these synonyms can help medical professionals discuss fetal membranes and related topics with greater clarity and precision.