Size discordancy between twins can be determined sonographically. hat said, diferences in crown-rump length are not reliable predictors for birthweight discordance (Miller, 2012). hus, most begin surveillance for discordancy ater the first trimester. One common method uses sonographic fetal biometry to compute an estimated weight for each twin (Chap. 10, p. 184). The weight of the smaller twin is then compared with that of the larger twin. hus, percent discordancy is calculated as the weight of the larger twin minus the weight of the smaller twin, then divided by the weight of the larger twin. Alternatively, given that abdominal circumference (AC) relects fetal nutrition, some use the sonographic AC value of each twin.
With these methods, some diagnose selective fetal-growth restriction if the AC measurements difer more than 20 mm or if the estimated fetal weight diference is 20 percent or more. That said, several diferent weight disparities between twins have been used to deine discordancy. Accumulated data suggest that weight discordancy greater than 25 to 30 percent most accurately predicts an adverse perinatal outcome. At Parkland, Hollier and coworkers (1999) retrospectively evaluated 1370 delivered twin pairs and stratiied twin weight discordancy in 5-percent increments within a range of 15 to 40 percent. They found that the incidence of respiratory distress syndrome, intraventricular hemorrhage, seizures, periventricular leukomalacia, sepsis, and necrotizing enterocolitis rose directly with the degree of weight discordancy. Rates of these conditions grew substantially if discordancy exceeded 25 percent. The relative risk of fetal death increased significantly to 5.6 if discordancy was more than 30 percent and rose to 18.9 if it was greater than 40 percent.
Sonographic monitoring of twin growth has become a mainstay in management. Monochorionic twins are generally monitored more frequently. his is because their risk of death is higher-3.6 percent versus 1.1 percent-and the risk of neurological damage in the surviving twin is substantial compared with those risks in dichorionic twins (Hillman, 2011; Lee, 2008). Thorson and colleagues (2011) retrospectively analyzed 108 monochorionic twin pregnancies and found that a sonographic evaluation interval > 2 weeks was associated with a higher Quintero stage at the time of TTTS diagnosis. hese indings have led some to recommend serial sonographic examination every 2 weeks in monochorionic twins (Simpson, 2013; Society for MaternalFetal Medicine, 2013). However, there have been no randomized trials of the optimal frequency of sonographic surveillance in monochorionic twin pregnancies. At Parkland Hospital, monochorionic twins undergo sonographic evaluation to assess interval growth every 4 weeks. In addition, a specific ultrasound examination to search for TTTS is completed at each intervening 2-week mark between these sonograms.
For dichorionic pregnancies, a recent report suggests that sonographic evaluation every 2 weeks would identiY more abnormalities prompting delivery (Corcoran, 2015). It has yet to be determined if this practice would improve perinatal outcomes. At our institution, dichorionic twins are sonographically evaluated every 6 weeks.
Depending on the degree of discordancy and the gestational age, fetal surveillance may be indicated, especially if one or both fetuses exhibit restricted growth. Nonstress testing, biophysical profile, and umbilical artery Doppler assessment have all been recommended in the management of twins. However, none has been assessed in appropriately sized prospective trials (Miller, 2012).