Fetal fractions
The temporal dynamic changes of fetal fractions by three different methods in all five pregnancies are shown in Figure 1, and detailed information at each timepoints listed in Table S1.
In patient 1, both fetuses were female and fetal fractiosn could not be calculated using Y-chromosome signals. However, the fetal fraction of the reduced T18 fetus was calculated using trisomy signals, showing an immediate decline from 3.27% to 2.01% after reduction and a mild increase to 3.41% at eight weeks after reduction. After that, the demised T18 fetus gradually stopped the release of cfDNA at 16 weeks after reduction as the fetal fraction dropped to close to 0. In contrast, after a transient decline, the overall fetal fraction maintained stable for several weeks, and then rise when the terminated fetus reduced the release of cfDNA at eight weeks after reduction.
In patient 2, who had a male euploidy and a male T18 fetus, the fetal fraction of the terminated T18 fetus increased from 1.80% to the maximum of 11.71% at nine weeks after reduction and then rapidly declined to close to 0 at 11 weeks after reduction. Since both fetuses were male, the male fetal fractions calculated by the Y-chromosome method and the overall fetal fractions calculated by FF-QuantSC were almost overlapped, which showed peak levels at nine weeks after reduction following the quick decline afterward. At 12 weeks after reduction when the terminated fetus stopped the release of cfDNA, the fetal fraction of the euploid fetus quickly rebound, and kept rising until birth. It was noted that at 15 and 20 weeks after reduction, the overall fetal fractions were slightly higher than the fetal fractions of male fetuses.
Both patient 3 and 4 had discordant sex twins with a female trisomy and a male euploid fetus. The fetal fractions of the terminated female fetus showed the increase of fetal fraction to peak levels at seven to nine weeks after reduction. After that, the terminated fetus in patient 3 gradually reduced the release of cfDNA and stopped discharge at 22 weeks after reduction, while the fetal fraction of the terminated fetus in patient 4 dropped to close 0 at 15 weeks after reduction. In both patients, once the fetal fractions of the terminated female fetuses began to decline, the fetal fractions of the euploid male fetuses rapidly increased, especially when the fetal fraction of the terminated female fetus was down to nearly 0. When the terminated female fetuses in both patient 3 and 4 still gave rise substantial amount of cfDNA to maternal circulation, the overall fetal fractions were higher than the fetal fractions of each co-twin. However, after the fetal fractions of the terminated female fetuses reduced to a relatively low level, the total fetal fractions were closely correlated with the fetal fractions of the euploid male fetuses.
In patient 5, since a male T18 fetus was terminated in the female-male twins, the fetal fractions calculated with trisomy signals and Y chromosome were almost identical, both showing a transient decrease shortly after reduction and later rise to maximum levels at eight weeks after reduction, and finally decline to nearly 0% at 14 weeks after reduction. Before seven weeks of post-reduction, the overall fetal fraction maintained two to four folds higher than the fetal fraction of the terminated fetus. Once the terminated fetus stopped the release of cfDNA at 14 weeks after reduction, the overall fetal fraction began to increase rapidly.