Pregnancy denial has a higher incidence than commonly supposed. So far psychological explanations rely on observations and interviews from individual cases. The presented case deals with a 13-year old mother with denied pregnancy, admitted to our department immediately after the birth of a healthy, mature infant. She was treated and observed interdisciplinary and for the first time we used psychometric testing in addition to medical and psychological treatments. Neither the psychological anamnesis nor test-psychometric results revealed a mental disorder. Some accentuated personality traits can explain the phenomenon to a small degree. However we adopt the idea of mental defence mechanisms, mainly projective identification, to explain the denied pregnancy in our case. The psychological processes of pregnancy denial thus become comprehensible and we derive suggestions for screening of at risk populations.