<P> The chorionic cavity is enclosed by the chorionic plate, which is composed of an inner layer of somatopleuric mesoderm and an outer layer of trophoblast cells . </P> <P> The mean sac diameter (MSD) can effectively estimate gestational age between 5 and 6 weeks, with an accuracy of about + / - 5 days . </P> <P> The yolk sac and embryo should be readily identifiable when the gestational sac reaches a certain size--a yolk sac should be seen when the gestational sac is 20mm and a fetal pole should be seen when the gestational sac reaches 25mm . </P> <P> Gestational sacs can be identified via ultrasound and are generally identified by the following 4 characteristics: </P>

Are gestational sac and yolk sac the same