Twin anemia–polycythemia sequence in sex‐discordant monochorionic dizygotic twins

Abstract

The possibility of monochorionic (MC) dizygotic (DZ) twins was first raised in 1970. As MC sex-discordant twins are rarely observed, MCDZ twins are seldom recognized clinically. Conception by assisted reproductive technology (ART) is associated with an increased frequency of MC diamniotic (DA) twins, with more than 50% of reported MCDZ twins conceived by ART1. With the increased use of ART, it is therefore reasonable to expect that the incidence of MCDZ twins may increase. MC twins share a single placenta and are connected by the placental circulation. They are at risk of transfusion imbalance in this circulation, which may lead to twin–twin transfusion syndrome (TTTS) or twin anemia–polycythemia sequence (TAPS). The diagnosis of TTTS at an early stage is based on discordant amniotic fluid volume. Currently, the diagnosis of TAPS is based on increased middle cerebral artery peak systolic velocity (MCA-PSV) in the donor twin (> 1.5 MoM) and decreased MCA-PSV in the recipient twin (< 1 MoM)2. Postnatally, TAPS can be diagnosed based on an intertwin hemoglobin difference of > 8 g/dL and at least one of the following criteria: (1) reticulocyte count ratio > 1.7; and (2) presence of only small vascular anastomoses on placental examination3. We report a case of TAPS diagnosed prenatally in a MCDZ twin pregnancy. A 32-year-old woman with a spontaneously conceived pregnancy was diagnosed with a MCDA twin gestation at 14 weeks by ultrasound. Sex discordance was noted on the fetal anatomy survey and was suggested on cell-free fetal DNA screening. Sex discordance was confirmed postnatally. A diagnosis of TAPS was suspected at 28 + 6 weeks based on MCA-PSV (1.65 MoM in the donor and 0.48 MoM in the recipient), and worsening of TAPS, with a new finding of placental echogenicity discordance between the donor and recipient sides, was suspected at 30 + 6 weeks (Figure 1). The patient was delivered at 31 + 2 weeks, following administration of corticosteroids and magnesium sulfate for neuroprotection, due to worsening of TAPS. TAPS was confirmed postnatally. Hemoglobin was 25.2 g/dL in the recipient twin and 7.5 g/dL in the donor twin. Reticulocyte count ratio (donor/recipient twin) was 14.7. Monochorionicity was confirmed by pathological examination. In TAPS, the placenta may have a discordant ultrasound appearance (hyperechoic on the donor twin side and hypoechoic on the recipient twin side) and a color difference on gross examination (pale appearance on the donor twin side and dark purple appearance on the recipient twin side)4. The current case demonstrated these findings (Figures 1 and 2). Although a case of TAPS in MCDZ twins was reported recently in a spontaneously delivered pregnancy in which the diagnosis was made postnatally5, we report what we believe to be the first case of TAPS in MCDZ twins that was diagnosed prenatally. Our report demonstrates that discordant MCA-PSV measurements and placental echogenicity differences in MC twin pregnancy are objective ultrasound findings that may indicate a developing and worsening status of TAPS in MCDZ twins. It is important to recognize that MCDZ twin gestations exist. Early determination of chorionicity during the first trimester with confirmation of fetal sex in the second trimester is the key in identifying a MCDZ twin gestation.