IVF conception: use the embryonic age from fertilization.Spontaneous conception: use the crown-rump length of the longest fetus at 11-13 weeks.Thus by 20 weeks only 85% of DC pregnancies demonstrate the λ sign. With advancing gestation, there is regression of the chorion laeve and the ‘lambda’ sign becomes progressively more difficult to identify.
In DC pregnancies there is a triangular placental tissue projection (λ sign) into the base of the In MC pregnancies there is no placental tissue projection into the base of the membrane (T sign). The best way to determine chorionicity by ultrasound at 11-13 weeks’ gestation is to examine the junction between the inter-fetal membrane and the placenta.In DC pregnancies the inter-fetal membrane is composed of a central layer of chorionic tissue sandwiched between two layers of amnion, whereas in MC pregnancies there is no chorionic layer.Therefore, all MC twins are monozygotic and 6 of 7 DC twins are dizygotic. One-third of monozygotic twins are dichorionic (DC) and two-thirds are monochorionic (MC). Two-thirds of twins are dizygotic (non-identical) and one-third monozygotic (identical).The risk of pregnancy complications is much higher in monochorionic (MC) than dichorionic (DC) pregnancies.Perinatal mortality and morbidity is higher with multiple pregnancies than singletons and therefore these pregnancies require additional support.Multiple births account for 2-3% of all live births and the incidence is increasing due to assisted conception and increasing maternal age.
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