Twin pregnancies.

Twin pregnancies, resulting in 2-3 % of all deliveries, are high risk pregnancies which need specialized care. A correct dating of pregnancy and the assessment of the chorionicity/amnionicity in the first trimester (< = 13 + 6 gestational weeks, GW) is essential for further monitoring. During first trimester risk stratification of monochorionic pregnancies includes evaluation of discordance of crown-rump-lengths, nuchal translucencies and amniotic fluid. At 16 GW sonographic risk evaluation of monochorionic twins involves differences in amniotic fluid and abdominal circumferences and detection of a velamentous cord insertion. A screening for fetal malformations with cervical length measurement as screening for preterm birth (cut-off < 25 mm) should be offered all twin pregnancies around 20 GW. In uncomplicated dichorionic pregnancies US examination should be performed every 4 weeks onwards to check fetal growth and amniotic fluid. An intertwin weight discordance > 20 % identifies pregnancies at increased risk of adverse outcome. Monochorionic pregnancies should be followed at least every two weeks for screening of twin-twin transfusion syndrome (TTTS), twin-anemia-polycythemia-sequence (TAPS) and selective fetal growth retardation (sFGR) with a start at 16 GW. The type 1-3 classification of sFGR in monochorionic twins depends on the pattern of end-diastolic velocity at the umbilical artery Doppler. The diagnosis of TTTS requires the presence of an oligyohydramnios (deepest vertical pocket (DVP) < 2 cm) in the donor twin and a polyhydramnios (DVP > 8 cm) in the recipient twin. However, the diagnosis of TAPS is based on the finding of discordant MCA Doppler values with a delta-MCA PV > 0.5 MoM.