smooth chorion

例句：

1. 1. The fetus is surrounded by a smooth chorion that protects it from the outside world.
   胎儿被一层平滑的羊膜包围，保护它免受外界影响。
2. 2. During the first trimester, the chorion begins to form a protective layer around the developing fetus.
   在怀孕的前三个月，羊膜开始形成一层保护层，保护着正在发育的胎儿。
3. 3. The chorion plays a crucial role in the development of the fetus, providing nutrients and protecting it from harmful substances.
   羊膜在胎儿发育中起着至关重要的作用，为胎儿提供营养并保护它免受有害物质的影响。