Yolk Sac: What Is It? Does Its Size Mean Anything?

Like your eggs scrambled, sunny side up, poached or boiled? Turns out we’ve been feeding on yolk since we were in utero – the yolk sac. The yolk sac helps the fetus to grow, but what exactly is it and when can it be seen through ultrasound? Is the size of the yolk sac affected by illnesses?

What is a yolk sac?

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A yolk sac is the sac in the uterus that is vital to the baby’s growth as it contains all the nutrients the baby needs for growth. When you first see it on your ultrasound scan, you might notice a small, coin-sized bag located near your tiny embryo. That’s it – your baby’s yolk sac.

The function of the yolk sac

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The yolk sac is, like the fetus, made from material originating from the fertilized egg. The implanted fertilized egg continues to undergo cell division, and the cells eventually are differentiated into the embryo and the yolk sac.

Why is there a need for a yolk sac if the mother is providing the baby with nutrients? The truth is, the mother doesn’t provide the baby with nutrients in the early stages of pregnancy. In fact, the mother can’t – doing so in the early stages of pregnancy will, scientifically speaking, put the baby in danger. The baby will receive nutrients directly from mom in time to come, but that is possible when a placenta is present. As the placenta is only completely formed in Weeks 13 to 17, your baby isn’t yet able to filter the nutrients coming from mom. Thus, your baby will get the nutrients they need from the yolk sac for the first few months of pregnancy.

Once the placenta is completely formed, the yolk sac has completed its life mission and is no longer needed. The placenta’s growth and the increase in size are inversely followed by the shrinking of the yolk sac. The yolk sac disappears in around Week 14.

If the yolk sac is absent, does that mean a miscarriage?

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Usually, a yolk sac is first detected together with the embryo in Week 7 of pregnancy. The presence of a yolk sac is an indication that the fetus is growing up well and that the risk of a miscarriage has decreased. However, if the first check-up past Week 7 shows that there is no yolk sac, it could mean that a miscarriage could happen. However, one thing to note is that the yolk sac is still very small, and the embryo might be positioned in front of it and hence cannot be seen on the ultrasound scan. You can always follow-up with another check-up, so don’t worry!

Is a bigger yolk sac a better yolk sac?


The yolk sac contains the nutrients that baby needs, so the bigger it is, the better it is? On the contrary, it is said that a “good”, “normal” yolk sac is a small yolk sac.

A big yolk sac might mean that the nutrients that are supposed to be sent to the baby aren’t being sent out properly, and as a result, are left stored in the yolk sac. There is a need to for you to go down for more check-ups if you have a large yolk sac. However, remember that people are all different and everything comes in different sizes, so if your doctor says that you have a big yolk sac but there isn’t anything to worry about, trust your practitioner on it.

You and your yolk sac are supporting baby’s growth

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A big yolk sac doesn’t necessarily mean that the fetus is sick. However, if you’ve had multiple miscarriages before, this could be an indication that your embryo isn’t getting all the nutrients necessary for growth. Follow-up with more checkups and ask your practitioner for the details.

The yolk sac is doing all it can for your baby – providing them with all the good nutrients and supporting their growth. What you can do is be, in turn, optimistic and be ready to carry on your pregnancy journey.