- Delta-NT or NT MoM: which is the most appropriate method for calculating accurate patient-specific risks for trisomy 21 in the first trimester? Spencer, K., Bindra, R., Nix, A.B., Heath, V., Nicolaides, K.H. Ultrasound in obstetrics & gynecology : the official journal of the International Society of Ultrasound in Obstetrics and Gynecology. (2003)