"From conception until the placenta is mature enough to nourish our babies, they are sustained with nourishment from this yolk sac. Once the placenta functions, the yolk sac generally is obliterated but on occasion there are remnants left behind to remind us of those very early days of prenatal life." ~ Patricia Edmonds
If you've ever had an early ultrasound one of the first things you might have been shown is the yolk sac which usually develops at around 5 weeks gestation. Doctor's can use it to gauge embryonic development by measuring it's size. I was lucky enough to see the remnants of one in a placenta I encapsulated and I wanted to learn more about it so I thought I'd share it with you. The literature contains a lot of medical terms which I have tried to explain as simply as possible so I hope I have them right. If you see anything that needs correcting please let me know!
The yolk sac is established during the second week of embryonic development. Some cells that form in the upper part of the yolk sac develop capillaries and manufacture blood cells for the embryo. The lower part of the sac develops tiny, finger-like projections called villi which penetrate the uterine wall absorbing food materials and oxygen to transport to the embryo.
As the embryo develops, it becomes larger than the yolk and begins to fold over it. Some of the yolk sac will continue developing in the embryo as parts of the gastrointestinal tract.
The yolk sac provides nourishment and acts as a circulatory system for your embryo until the placenta develops and takes over these functions. It contributes to the development of your baby's gastrointestinal and reproductive systems.
By the time of birth, the yolk sac has usually disappeared or it may be visible as a small oval between 1-6mm between the amnion and chorion (membranes) on the placenta - see below.
Kelly Harper is a self-confessed "placenta nerd". She loves learning about all things pregnancy, birth and baby related and delights in sharing that information with her doula and encapsulation clients.