[Multiple pregnancy at 11+0 - 13+6 weeks].

Chorionicity, rather than zygosity, is the main factor determining pregnancy outcome. In monochorionic twins the rates of miscarriage, perinatal death, preterm delivery, fetal growth restriction and fetal abnormalities are much higher than in dichorionic twins. High mortality confined to monochorionic pregnancies is the consequence of severe early-onset TTTS. Death of a monochorionic fetus is associated with a high chance of sudden death or severe neurological damage in the co-twin. Sonographic examination provides reliable distinction between dichorionic and monochorionic pregnancies. Reduction of the excess fetal loss in twins, can only be achieved through early identification of monochorionic pregnancies by ultrasound examination at 11+0 - 13+6 weeks, close surveillance and appropriate treatment in those that develop severe TTTS. In multiple pregnancies, compared to singletons, prenatal diagnosis of chromosomal abnormalities is complicated because, firstly, the techniques of invasive testing may provide uncertain results or may be associated with higher risks of miscarriage and, secondly, the fetuses may be discordant for an abnormality.