NIPT results
As a result of the persistence of a high fetal fraction of demised trisomy co-twin, NIPT T-scores of the remaining euploid fetuses in all five pregnancies were compromised for long periods after reduction (Figure 2a). Detailed NIPT results of each time points are listed in Table S1.
Patient 1 had her T-score of chromosome 18 reduced from 7.27 before reduction to 4.55 at three weeks after reduction, and later rise to 8.20 at eight weeks after reduction. Then her T-score declined to 1.24 at 16 weeks after reduction and her NIPT results remained T18 low-risk afterward.
Patient 2 had her T-score of chromosome 18 dramatically rise from 4.55 at two weeks after reduction to 24.62 at nine weeks after reduction, and then rapidly declined to 2.09 at 12 weeks after reduction for T18 low-risk afterward. Interestingly, her T-score of chromosome 13 reached 3.33 at 15 weeks after reduction, and then rise to 5.65 at 20 weeks after reduction, indicating high risk of trisomy 13 (T13) (Figure 2b). This was consistent with the results of her fetal fraction, showing that at 15 and 20 weeks post-reduction, the overall fetal fraction calculated by FF-QuantSC was slightly higher than the fetal fractions of both male fetuses calculated by the Y-chromosome method. A later confirmation in placenta tissues by sequencing placental tissues validated the presence of placenta mosaicism of T13 (Figure 2c).
In patient 3, in the first two weeks after reduction, her T-score of chromosome 21 only mildly increased from 10.04 to 13.94. After that, her T-score reached a maximum of 36.60 at eight weeks after reduction, and then quickly declined to 8.76 at 12 weeks after reduction. Since then, her NIPT results maintained high risk for T21 for a very long time until 22 weeks after reduction.
In patient 4, her T-score of chromosome 18 also showed an initial mild but later faster increase, and reached a maximum of 17.27 at seven weeks after reduction. After that, the T-score gradually declined to 1.85 at 15 weeks after reduction. Since then, the NIPT results changed to T18 low risk.
Patient 5 had her T-score of chromosome 18 reduced from 12.39 before reduction to 4.84 at two weeks after reduction, and rise to 16.04% at seven weeks after reduction. The T-score then continuously declined, and the NIPT results changed to T18 low-risk at 14 weeks after reduction.