The differential diagnosis of fetal neck mass includes fetal goiter, cervical teratoma, cystic hygroma and lymphangioma 

• cervical teratomas are heterogeneous (solid cystic) in echotexture, may show calcification and usually are very large. Teratoma's usually occurs in the antero-lateral part of neck.
• cystic hygromas are usually cystic lesion with septations and usually are present posterior to the neck 
• lymphangioma also has a cystic appearance
• goiter is well defined, solid homogeneous mass, present at anterior part of neck

An antenatal diagnosis of a fetal goiter was made due to hypothyroidism. This was confirmed by postnatal clinical and lab examination of baby.