Aims Primary histopathology of miscarriage remains undetermined in the majority of cases. This study was conducted to determine histological characteristics pertinent to miscarriage.

Methods The study groups were composed of elective abortions (n=29) and miscarriages (n=45) comprised of chromosomally normal (n=15) and abnormal cases (n=30). Immunohistochemistry was done against CD3, CD8, TIA-1 and CD56.

Results Two histological features—diffuse decidual leucocytoclastic necrosis (DDLN) and decidual natural killer cell aggregates (NKCA)—were relatively common in miscarriages. The frequencies of DDLN and NKCA were different between the groups (p<0.05 and p<0.05, respectively). DDLN was found in 13.8% (4/29) of elective abortions, while it was observed in 60.0% (9/15) and 23.3% (7/30) of chromosomally normal and abnormal miscarriages, respectively. DDLN was more frequent in chromosomally normal miscarriages than in elective abortions (p=0.004). NKCA was present in 13.8% (4/29) of elective abortions, while being found in 33.3% (5/15) and 43.3% (13/30) of chromosomally normal and abnormal miscarriages, respectively. NKCA was more frequent in chromosomally abnormal miscarriages than in elective abortions (p=0.020).

Conclusions The findings strongly suggest that defective placentation and abnormal maternal immune response are associated with miscarriage. DDLN and NKCA seem to have diagnostic values in the pathological evaluation of miscarriage.