Zygosity of twins: is it time for a new terminology?

The term zygosity reflects the origin of twins--when twins are monozygotic, they are derived from a single fertilized ovum; when twins are dizygotic--the come from two fertilized ova. Zygosity assessment seemed straightforward 50 years ago. Currently, as more information and more technology becomes available, zygosity testing becomes a real problem. It is important to realize the importance of correct zygosity testing not only in the antepartum care of the pregnant mother, but more importantly--during the whole lifetime of twin individuals. This paper discusses revolving around zygosity, methods of assessment, their limitations and proposes new terminology which incorporates the recent knowledge about early human development.