Intertwin differences in umbilical venous diameter and flow reflect placental sharing and perinatal outcome in monochorionic twins with selective fetal growth restriction.

BACKGROUND: In monochorionic diamniotic (MCDA) twin pregnancies with selective fetal growth restriction (sFGR), antenatal risk stratification relies primarily on umbilical artery (UA) Doppler assessment of the smaller twin. However, UA patterns may change, and placental sharing cannot be measured accurately. Umbilical venous (UV) diameter and flow may reflect each twin's oxygenated blood supply more directly. OBJECTIVES: To evaluate if intertwin differences in UV diameter and flow provide information on placental sharing and perinatal outcomes, in addition to established ultrasound markers. STUDY DESIGN: Prospective single-center cohort study of MCDA pregnancies with sFGR, (defined as estimated fetal weight discordance (EFW) of ≥20%). At diagnosis, intra-abdominal and free cord loop UV diameter and intra-abdominal UV flow, EFW, UA Doppler of the smaller twin, and cord insertion type were assessed. Placental sharing was determined by postnatal angiography. Adverse outcome was defined as fetal demise, neonatal loss, or birth <34 weeks of gestation. Associations were examined using uni- and multivariable regression analyses. Receiver operating characteristic analyses compared models with and without UV parameters. RESULTS: Of 114 sFGR pregnancies, 99 (87%) had placental angiography. Intertwin differences in UV parameters were strongly associated with placental sharing. After adjustment for EFW discordance, cord insertion type, and the smaller twin's UA Doppler pattern, the UV parameters remained independently associated with placental sharing. Intertwin differences in intra-abdominal UV diameter and flow were also independently associated with adverse outcomes. Adding UV parameters to models that included the smaller twin's UA Doppler pattern led to modest but consistent improvement in discrimination for unequal sharing and adverse perinatal outcome. This improvement was most pronounced for the UV flow with an area under the receiver operating characteristic curve (AUC) of 0.89 (0.82-0.96) versus 0.79 (0.70-0.88) (P=0.02) for unequal sharing and 0.82 (0.74-0.90) versus 0.74 (0.66-0.81) (P<0.01) for adverse outcomes, with and without the addition of UV flow, respectively. CONCLUSION: In MCDA twin pregnancies with sFGR, intertwin differences in UV diameter and flow reflect placental sharing and are independently associated with perinatal risk. These findings suggest that UV parameters may complement established ultrasound markers in the assessment of placental sharing and risk stratification.