Discussion
In this study, we assessed the association between fetal fraction and birth weight discordant and sIUGR. It is showed that fetal fraction was positively correlated with the difference of birth weight. For every one percent increase in fetal fraction, the difference of birth weight between twins increases by 0.4 percent. Higher fetal fraction was significantly associated with the increased risk of birthweight discordance of 20% and 25% and sIUGR. 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% among these with fetal fraction of ≥11.790, and significantly increased risks of birth weight discordance of 25% and sFGR among these with fetal fraction of ≥14.800.
Cell-free fetal DNA actually originates from placenta[11].  Throughout a typical pregnancy, cffDNA experiences a gradual increase, comprising roughly 13% of all cell-free DNA found in maternal plasma towards the end of gestation, before swiftly decreasing to undetectable levels after childbirth[12, 13]. The size of the placenta and the rate of trophoblast apoptosis are factors that affect the release of cffDNA[9]. According to reports, cffDNA triggers the secretion of type 1 interferons such as IFN-β and IFN-α, along with other pro-inflammatory agents[14]. The introduction of CpG resulted in gestational hypertension and heightened vasoconstriction in rats. This suggests that cffDNA has the ability to initiate inflammatory cascades as a pro-inflammatory trigger.
Research has demonstrated that oxidative stress results in trophoblast apoptosis and elevated cffDNA release[15]. The liberation of cffDNA can be affected by doxorubicin and high-mobility group box protein-1, which are inflammatory agents. Conversely, lipopolysaccharide does not affect the release of early or term cffDNA[16], suggesting that cffDNA is involved in sterile inflammation as opposed to infectious processes. Sterile inflammation may lead to placental dysfunction and consequently cause pregnancy complications.
In order to predict negative outcomes earlier and non-invasively, many studies have linked pregnancy complications with cffDNA plasma concentrations. Pregnancies affected by pre-eclampsia, preterm delivery, and FGR were found to have higher cffDNA concentrations during the second trimester. Additionally, there was a noticeable increase in cffDNA levels three weeks prior to the onset of symptoms[17]. In twins with birth weight discordant, we were able to see an increase in cffDNA which was represented by fetal frection. Increased levels of cffDNA are also a result of intrauterine surgeries, such as laser ablation, used to treat twin-twin transfusion syndrome[18]. It is possible that the rise in cffDNA levels observed in twins with discordant birth weight and selective fetal growth restriction (sFGR) is a result of the repair process. In cases where there is discordant growth in DC pregnancies, the incidence of preeclampsia can be as high as 37.5%, which is significantly greater than the 21.9% rate observed in MC sFGR pregnancies[19, 20]. It is known that placental insufficiency affects only one fetus and results in selective growth restriction in DC pregnancies[10]. After corrected for both preeclampsia and gestational diabetes, increased cffDNA is still associated with the incidence of birth weight discordance and sFGR, which showed that there may be other pathological mechanisms involved. Placentae that are affected by uneven sharing have fetal circulations that are more tightly connected with larger and more frequent arterio-arterial (AA) anastomoses. This can be helpful for growth-restricted fetuses, as it allows their co-twin to compensate to some extent for the inadequacy of their own placenta. However, this close connection also poses risks to appropriately grown fetuses, who are vulnerable to sudden changes in the blood pressure of their smaller co-twin. Although unequal placental sharing is the primary cause of sFGR in MC pregnancies, the interdependent feto-placental circulations play a crucial role in the prognosis and perinatal outcome. In cases of sFGR, especially those with abnormal umbilical artery Doppler, glucose transporters (GLUTs) were found to be elevated due to hypo-perfusion. We suspect that increased cffDNA in MC twins with sFGR is a result of apoptosis in such hypoxic environments. Additionally, hypermetabolism may also be a factor in elevated cffDNA levels since the placenta attempts to increase glucose transport to the fetal circulation as an adaptive response[21]. We suspect that increased cffDNA in MC twins with sFGR is a consequence of apoptosis in such hypoxic environment. Further, hypermetabolism may also be a reason for elevated cffDNA since the placenta attempts to increase glucose transport to the fetal circulation as an adaptive response.