The gestational sac is known to be something that is contained within the uterus. This can help determine if a woman is pregnant even before the embryos starts to appear. The yolk sac is something that can be seen in the gestational sac. It is said to be an anatomical structure that you will first see if you would check the gestational sac. Take note that these two can be very small or can hardly be seen even in an ultrasound.
There are times when they have to be artificially colored so that they can be seen. Take note that it is not the only the size of the yolk sac that will determine if the woman had a miscarriage. Other factors should be checked too.