The thymus is a central organ of immune system important for the development and maintenance of the cellular immunity. Thymus reaches the biggest size compared to the body proportions during early postnatal period of babies. Abnormal fetal growth during the intrauterine period may also cause disturbances of the thymus development and prenatal growth. The abnormal thymic development can result in its insufficient function postnatally. The thymus size of newborns can indirectly reflect the immune systemcondition and general susceptibility to diseases during postnatal life of children. During the prenatal and postnatal development the thymus is sensitive to many environmental factors such as an acute and chronic stress, malnutrition, infections or hormonal therapy. On the other hand the consumption of maternal milk has a positive effect to the postnatal growing and function of the thymus. The present work brings a review of factors which could have effect to the thymus development, size and involution. The studies of relationship between the thymic size and the anthropometric parameters (head and armcircumference, birth weight and length) of physiological newborns are also demonstrated.

        Key words: stress-related thymus involution, ultrasonography of newborns, anthropometry of newborns