If genetic factors are important, concordance in monozygous twins, who are 100-percent genetically identical, will be greater than in dizygous twins, who are only 50-percent genetically identical.
From twin studies, the concordance rate is estimated at 57 percent for monozygous twins and 14 percent for dizygotic twins.
The incidence of multiple births increases with the age of the mother but distinctly more so for dizygous rather than for monozygous twins and more so still for triplets.
Derivatives
monozygosity
noun
Both fetuses were female, consistent with the monozygosity diagnosed on placental examination.