The association of Fetal fraction with birthweight discordance and sIUGR
As shown in Table 2, the adjusted model demonstrated that fetal fraction was positively correlated with the difference of birth weight (β=0.004, 95%CI: 0.001~0.006). That is, for every one percent increase in fetal fraction, the difference of birth weight between twins increases by 0.4 percent.
The results of logistic analysis were shown in Table 3, indicating that higher fetal fraction was significantly associated with the increased risk of birthweight discordance of 20% (adjusted OR:1.073, 95%CI: 1.009~1.142) and 25% (adjusted OR:1.092, 95%CI: 1.006~1.185) and sIUGR (adjusted OR:1.130, 95%CI: 1.038~1.231).
The optimum cut-off points for fetal fraction in birthweight discordance and sIUGR were obtained with a ROC analysis (Table 4). Using ROC analysis, we obtained the optimum cut-off point of fetal fraction ≥ 11.790, ≥ 14.800 and ≥ 14.800 for birthweight discordance of 20% and 25% and sIUGR, respectively. For birthweight discordance of 20%, the AUC was 0.591, the sensitivity was 52.9% and the specificity was 66.3%(p =0.017). The AUC of sIUGR was 0.620, with 37.5% sensitivity and 86.5% specificity(p =0.024).
Compared with women with fetal fraction of < 11.790, there were a 1.091-fold higher risk of birth weight discordance of 20% (adjusted OR: 2.091, 95%CI: 1.218~3.591) among these with fetal fraction of ≥11.790, and significantly increased risks of birth weight discordance of 25% (adjusted OR: 3.045, 95%CI: 1.297~7.149) and sIUGR (adjusted OR: 3.526, 95%CI: 1.443~8.618) among these with fetal fraction of ≥14.800.