The jugular lymphatic sacs (JLS) are a physiological and temporary part of fetal lymphatic development. They are formed from small buds of lymphatic endothelial cells arising from the internal jugular veins. By 14 weeks gestation they have developed into lymphatic nodes which drain into the systemic circulation. Visibility of JLS on ultrasound past 14 weeks of gestation implies distension and this may be associated with raised nuchal translucency. [from
HPO]