3-Week Embryo Development: Size, Structure & Key Features

At 3 weeks, an embryo is about the size of a poppy seed.

It consists of two layers of cells:

- The inner cell mass, which will eventually become the embryo itself.

- The outer layer of cells, which will form the placenta.

There is also a fluid-filled cavity within the embryo called the amniotic sac. The amniotic sac provides protection and nourishment for the embryo as it develops.

At this stage of development, the embryo is not yet recognizable as a human being. However, it is undergoing rapid growth and change, and will soon begin to take on the distinctive features of a human fetus.