In a monozygotic twin pregnancy, the expectation is that both twins are affected or both are unaffected, but in a dizygotic twin pregnancy, whether one twin is affected will be unrelated to whether the other is affected. Because of the close association between chorionicity and zygosity, and because monozygotic twins are less common, it is reasonable to adopt the following approach (1):-
If the chorionicity is unknown, αlpha assumes that the pregnancy is dichorionic.
In a dichorionic pregnancy, αlpha estimates the risk separately for each twin, based on half the maternal age-specific risk (adjusted for the prevalence of Down's syndrome in dichorionic pregnancies (2)) , and the NT measurement for each twin. The risk for the pregnancy is then the sum of the two twin risks.
In a monochorionic pregnancy, αlpha calculates the risk for the pregnancy based on the woman's age (adjusted for the prevalance of Down's syndrome in monochorionic pregnancies (2)) and the average of the two NT MoMs, because in expectation each twin will have the same risk.
If biochemical markers are measured, the risk based on the NT measurements and the maternal age becomes the new background risk for the pregnancy, and αlpha combines this in the usual way with the biochemical marker levels to give a Pseudo-Risk for the pregnancy.
1. Prenatal screening for Down syndrome and neural tube defects in twin pregnancies (2005)
2. Prevalence and risk of Down syndrome in monozygotic and dizygotic multiple pregnancies in Europe: implications for prenatal screening. (2014)