Table 5.
Fetal fraction (FF) estimates in six twin pregnancies with confirmed trisomy 21 and at least one male fetus, included in Clinical Study B
| Fetal karyotype (Twin A/B) | FF by X (%) | FF by Chr 21 (%) | Source of FF | Correlation with clinical findings |
|---|---|---|---|---|
| XY/XY + 21 | 31.9 | 12.0 |
FF by X is from Twins A and B FF by Chr 21 is from Twin B |
Expectation: higher FF by X*
Finding: higher FF by X |
| XY/XY + 21 | 13.5 | 6.4 |
FF by X is from Twins A and B FF by Chr 21 is from Twin B |
Expectation: higher FF by X*
Finding: higher FF by X |
| XX/XY + 21 | 11.3 | 4.8 | Both FF measurements are from Twin B |
Expectation: similar FF†
Finding: higher FF by X‡ |
| XX/XY + 21 | 6.7 | 7.2 | Both FF measurements are from Twin B |
Expectation: similar FF†
Finding: similar FF |
| XY/XX + 21 | 6.6 | 9.6 |
FF by X is from Twin A FF by Chr 21 is from Twin B |
Expectation: FF within 1.5 of each other§
Finding: FF within 1.5 of each other |
| XX/XY + 21 | 15.1 | 14.8 | Both FF measurements are from Twin B |
Expectation: similar FF†
Finding: similar FF |
*
FF by X is combined contribution from both twins and FF by chromosome (Chr) 21 is only from affected twin.
†
Because both FF measurements are from same twin, we would expect similar values for each method.
‡
Originally triplet pregnancy with one demise at around 6 weeks; if the demised twin was male, without trisomy 21, this could account for higher FF by X.