2. Yolk sac is measured by placing calipers on the inner limits of the longer diameter. It usually appears at the periphery of the gestational sac and should not be floating within the sac. Size of the sac, shape, echogenicity of the rim and center of sac, its number and degenerative changes such as calcification will be evaluated. YS having diameter between 3-7mm, rounded shape, absence of degenerative changes, presence of echogenic rim and hypoechoic center are considered normal. Any deviation from above parameters is considered abnormal.