[Discordant anomaly in twin pregnancy].

The risk of fetal loss is multiplied by 4 in monochorionic pregnancies by comparison with dichorionic one. The incidence of selective growth retardation (sIUGR) is between 12.5 to 25%. This variation is explained by the mixture between pure twin-to-twin transfusion syndrome (TTTS) and pregnancies without TTTS but with a pure sIUGR. The ability to diagnose prenatally sIUGR is low; the positive predictive value is only 37% (EL4). The umbilical flow velocity waveforms with absent diastolic flow (Type II) or with diastolic intermittent flow (Type III) is clearly a strong risk factor of mortality of the IUGR and also a risk factor of leucomalacia of the bigger fetus (EL3). The observed incidence of malformations in twins is 4.05% versus 2.38% for singletons (OR=1.7 [IC 95% 1.47-1.97]). Furthermore, the rate of fetal malformations is higher in monochorionic pregnancies by comparison with dichorionic one, 6.33% versus 3.43% (OR=1.8 [IC 95% 1.3-2.5]) (EL3). In the majority of the cases, the malformation is concerning only one fetus. The most frequent malformations are those of the central nervous system by comparison with singletons, those of the urinary tract and the cardiovascular malformations. Monozygotic pregnancies are not necessarily phenotypically and genetically identical. In situation of asymmetrical malformation, it is necessary to propose fetal karyotype of the malformed fetus. In case aneuploidy of the malformed fetus, secondarily it will be necessary to explore the other apparently normal one. Uniparental disomy should be suspected in such situation (EL3). In TRAP sequence, an intensive follow up should be organised with serial measurement of the respective size of the acardiac fetus and the normal one, Doppler exploration should be done to look for early sign of cardiac decompensation (expert viewpoint). In dichorionic pregnancies, when the malformation is threatening the whole pregnancy like anencephaly, a selective feticide allow a prolongation of the pregnancy, but with a risk of fetal loss of 8% and a risk of iatrogenic prematurity of 12%. In such situation, it could be wise to evaluate the evolution or to wait until the third trimester to undergo the procedure (EL3). In monochorionic pregnancies, in the same situation of anencephalic fetus the best option is bipolar cord coagulation. The result seems to be better after 18 weeks of gestation (EL4). There is a 20% risk of premature rupture of membrane. In the up to date analysis of the literature, there is no formal indication of selective feticide expect case of TRAP sequence with cardiac decompensation of the normal fetus. The very special situation of sIUGR is the object of a randomized trial. In all cases the active participation of the patients to the therapeutic option is mandatory (expert viewpoint).