As soon as the egg is fertilized by the sperm, this marks the beginning of a new life. Along with this new condition, several other temporary organs also begin to form, and one of them is the yolk sac. The yolk sac is crucial for the proper formation of the embryo during the first weeks of pregnancy1. But do you know what the yolk sac is and what it does? And what is its role in the formation of the baby, and how long does it remain available? Let’s find out!
How does it form?
With the fertilization of the egg and the formation of a new cell nucleus, the zygote moves toward the uterus. Along the way, cell multiplication happens rapidly, dividing more and more. Upon reaching the uterine cavity, the newly formed mass of cells divides again, this time into two parts. One of them will become the baby, and the other part will be the support this new life will have during pregnancy. The second part will attach to the uterus, settle, and create a sort of root to extract from the endometrium all the blood needed for the initial vascularization of the baby. Implantation is a delicate stage, and can cause mild pain and even bleeding; however, it is also possible not to feel anything out of the ordinary. When implantation occurs, development accelerates. What will become the baby starts to form quickly, giving rise to the neural tube2. Meanwhile, the other part becomes a kind of balloon, turning into the yolk sac. Video with yolk sac at 6 weeks
What does it do?
Just like the placenta, the yolk sac provides oxygen, nutrients, and blood to the embryo, but with one difference: it is temporary, as it remains only until the placenta reaches the appropriate size and maturity to take over its role during pregnancy. Stimulated by the HCG hormone, it grows much more quickly and, even before the baby can be seen, it is already working. The appearance of the yolk sac is quite specific and distinctive; it looks like a gas balloon, but much smaller, in proportion to the stage of pregnancy. It can be seen on ultrasound from 5 weeks, together with the gestational sac. The size of the yolk sac is much larger than the baby at this stage. For this reason, it is possible not to see the baby, but only the yolk sac during the first exam. As the weeks pass, the baby becomes larger than the yolk sac and will then appear on the ultrasound scan.
How Long Does the Yolk Sac Remain Active?
Typically, the yolk sac remains active until around 10 weeks of pregnancy, give or take a little. This is because the placenta must be the correct size and have the ideal appearance in order to fully perform its function. Right after the placenta takes over the baby’s nourishment, the yolk sac becomes inactive and shrinks over the following days. It is not necessary to worry about the yolk sac after the placenta begins to function, as the placenta will take over quickly, without any need to interrupt the vascular support that was previously constant.
Problems With the Yolk Sac
Some abnormalities can be detected in the first ultrasound exam. One of them is the increase in the size of the yolk sac, or hydropic yolk sac3. If it is larger than expected (between 3 and 5mm at 6 weeks), there may be a problem with fetal development. However, this is not a rule, just a possibility that the doctor should investigate within a few days with a new imaging exam. Normally, the yolk sac is larger than the embryo, but it should not exceed the size limit of 1 to 2, that is, it should not be more than twice the size of the baby in the early stage. If this happens, it is important to investigate what is going on: if there is excessive hormonal stimulation or if there is some genetic problem causing fetal malformation. See also: What is a Gestational Sac and What is the Correct Size During Pregnancy
