the number of chorionic membranes present on direct examination of the placenta and membranes following delivery of *twins. Chorionicity can be determined antenatally by ultrasound, ideally between 10 and 14 weeks gestation. Where two placental masses are identified, the pregnancy is dichorionic. More frequently a single placental mass exists and chorionicity can only be determined by evaluation of the intertwin membrane; the pregnancy is monochorionic when the fetuses share a chorion (see illustration; see also lambda sign; T-sign). Chorionicity rather than zygosity is a better determinant of perinatal outcome in twins. The perinatal mortality rate of monochorionic pregnancies may be up to four times that of dichorionic.
pl. n. two individuals who are born at the same time and of the same parents. Fraternal (or dizygotic) twins are the result of the simultaneous fertilization of two egg cells; they may be of different sexes and are no more alike than ordinary siblings. Identical (or monozygotic) twins result from the fertilization of a single egg cell that subsequently divides to give two separate fetuses. They are of the same sex and otherwise genetically identical; any differences in their appearance are due to environmental influences. All dizygotic twins and about one-third of monozygotic twins are dichorionic (i.e. they develop within separate chorionic membranes). About two-thirds of monozygotic twins are monochorionic, sharing a chorion. See chorionicity. See also conjoined twins.... twins
(TTTS) a condition in which communicating vessels in the shared placenta of monochorionic twins (see chorionicity) divert blood to one fetus (the recipient) from the other (the donor), resulting in one fetus with increased blood volume and one anaemic fetus. It complicates 15% of monochorionic twin pregnancies, and a system of ultrasound staging has been developed to assess the severity of the syndrome. TTTS is associated with a high perinatal mortality rate. There is significant morbidity and poor neurodevelopmental outcome in surviving infants due to complications of the disease itself and the high preterm birth rate that invariably accompanies this condition. A range of treatments, including *amnioreduction, *septostomy, and laser ablation of the communicating vessels, have led to an improvement in overall perinatal survival rates.... twin-to-twin transfusion syndrome
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