From the moment a woman’s egg is fertilized, and for the first few weeks of development -- before many women even realize that they are pregnant -- cells are forming and organizing in complex patterns.

In the very beginning of pregnancy (when the fertilized egg is called a zygote) there are two kinds of cells that grow; those that will become the baby, and those that will create the surrounding system that is designed to contain, nourish, and protect the baby for the next 40 or so weeks of pregnancy.

During those very first weeks of development the human embryo goes through stages that are almost more fish or bird-like than human. If you can imagine the woman’s uterus as being the hard shell of an egg, the quickly reproducing cells at the beginning of pregnancy will form into the embryo as well as the parts that make up the inside of the egg, and then these parts will develop into what we recognize as the placenta, umbilical cord, and amniotic sac.

This is the anatomy of a chicken’s egg, but in the early days of fertilization there are many similarities. As the little fertilized egg cells multiply and develop into more and more cells, they form four extraembryonic membranes called the chorion, amnion, allantois, and yolk sac. (Extraembryonic is science-speak for “outside the embryonic body.”)

The Chorion will become most of the placenta, as well as the outer layer of the two membranes that hold the baby and amniotic fluid until birth. In the very early days of pregnancy, there are actually many more cells that are assigned to develop into the chorion than into the actual embryo.

The Amnion will become the inner layer of those two membranes, the layer that actually holds the amniotic fluid. If you can again picture the uterus as the kind of egg you might eat for breakfast, this would be like the membrane inside the shell that sometimes doesn’t quite open up when you crack the egg over a bowl.

The Allantois will become most of the umbilical cord and a portion of the placenta. The allantois acts as a kind of shunt for removing waste products. In animals that lay eggs this exists to separate the waste products from the embryo; in humans it develops further into a more sophisticated umbilical system to support the much longer gestation.

The Yolk sac provides nourishment and basic circulation as it acts in the role of the placenta, while the placenta is still forming. It will become the portion of the umbilical cord that is attached to the baby’s abdomen, developing along with the allantois to form a complete umbilical cord that is attached to the embryo on one end and to the placenta on the other end.

Since they all originate from the same fertilized egg, the embryo and the placenta and umbilical cord develop as one little bundle in the beginning, but about three weeks after fertilization the embryo begins to separate from the tiny, developing placenta, remaining connected via a connective stalk.