The junctional zone (JZ) is an internal layer of the myometrium, with unique characteristics that allow its visualization as a hypoechogenic structure in three-dimensional (3D) ultrasound. While there is a growing realization that the origins of major obstetrical complications associated with defective deep placentation, such as pre-term labour, fetal growth restriction and pre-eclampsia, may lie in the very early pregnancy events, the underlying mechanisms are not understood.