The yolk sac is the earliest visible portion of the gestational sac during early pregnancy. It is a critical landmark and is usually visible at five weeks of pregnancy. The gestational sac is the only intrauterine structure that can be used to determine if a pregnancy exists, that is ,until the embryo is developed enough to be identified. The first month following conception, which involves the combining of the sperm and the egg, there is a lot ofactivity going on for the developing little embryo! Although nothing much can be seen on the outside, at a cellular level there is a flurry of activity.
During the first week of pregnancy, the fertilised egg begins to divide at a rapid rate, splitting into two, then four, eight, and 16 cells and continues to do so until it has become a hollow ball of a few hundred cells.
When the hollow ball implants itself int he wall of the womb on about the sixth day, it begins to thicken at the base. During the second week, the tissue that will become the placenta begins to establish a link with the mother’s blood supply. Once this happens the embryo will be able to absorb more building supplies and grow.
The yolk sac in early pregnancy is a source of nourishment for the developing baby before it is absorbed by the baby as part of the gut. It functions as the developmental circulatory system of the human embryo before internal circulation begins. At approximately the end of the fourth week, the yolk sac has become connected to the primitive digestive system, which allows the yolk sac to provide nutrients to the embryo. It serves as the baby’s factory for blood cells until the liver, spleen and then the bone marrow can take over the job.
The amniotic sac which will eventually surround the baby completely is, at this stage, just a small bubble. The bottom section of the cells is destined to become the layer of “inner skin”, the lining of the throat, the stomach, the intestines, lungs, and various internal organs. In the top layer, next to the amniotic sac, are the cells that will grow to become most of the body. The yolk sac will disappear as the placenta grows to meet the needs of the developing baby.