Discordance in maximum velocities in the middle cerebral arteries of monochorionic twins: beyond twin-anemia-polycythemia sequence.

BACKGROUND In monochorionic pregnancies, elevated middle cerebral artery (MCA) peak systolic velocity (PSV) in one twin is a key marker of fetal anemia, notably in TAPS, but may also occur spontaneously in sIUGR, owing to brain-sparing mechanisms or in the context of TTTS particularly following laser therapy. Distinguishing between these etiologies remains challenging, complicating clinical management and counseling. In addition, current MCA-PSV charts are primarily derived from singleton pregnancies, with limited data on monochorionic twins, especially before 18 weeks of gestation. OBJECTIVES The aim of this study was twofold: first, to establish a dedicated MCA-PSV chart for monochorionic twins, and second, to propose a new hemodynamic index (MCA-PSV/UA-PI ratio) for differentiating MCA-PSV changes due to hemodynamic adaptation from fetal anemia. STUDY DESIGN All consecutive monochorionic twin pregnancies referred to our tertiary reference center between January 2019 and December 2023, either for standard follow-up or for a specific complication, were included. This study was conducted in two phases : first, the development of gestational age-specific charts for MCA-PSV and the MCA-PSV/UA-PI ratio using data from uncomplicated pregnancies ; second, the evaluation of the hemodynamic index in pathological conditions associated with elevated MCA-PSV values (sIUGR, spontaneous TAPS, and post-laser follow-up). A Z-score of 2.32 (corresponding to the 99th percentile) was used as the threshold for pathological values. RESULTS Overall, 810 monochorionic twin pregnancies were included. 389 patients with uncomplicated pregnancies were used to generate gestational age-specific charts for both MCA-PSV and MCA-PSV/UA-PI ratio. With a total of 4,021 observations of MCA-PSV, including 640 observations between 14 and 18 weeks of gestation, the estimated mean MCA-PSV before 18 weeks significantly differed from both singleton and twin charts. Differences between observed and smoothed mean MCA-PSV were small (0.49 cm/s, SD ± 0.34). We investigated pathological conditions associated with elevated MCA velocity. Among 67 severe sIUGR cases, 40% presented elevated MCA velocity. Their MCA-PSV/UA-PI ratio was significantly different from spontaneous TAPS (mean MCA-PSV/UA-PI ratio Z-score of 0.60 vs 4.16 for sIUGR and TAPS, respectively, p<0.001): The ratio Z-score was within the normal range for 93% of sIUGR cases and for none of the spontaneous TAPS. Post-laser follow-up showed MCA-PSV elevation in 15/281 TTTS cases (5%), with 9/15 diagnosed as post-laser TAPS. No clear correlation was found between Doppler indices and post-laser TAPS. CONCLUSION This study provides a new gestational age-specific MCA-PSV chart and introduces a novel hemodynamic index (MCA-PSV/UA-PI ratio) that may help differentiate spontaneous anemia from hemodynamic changes and thus, refine both diagnosis and prognosis of complicated monochorionic pregnancies, especially in early gestational ages.