Discussion
For the first time, in our study, we found that the gestational age at sIUD is associated with PTB after sIUD in MC twins. We found that the 25-30 weeks at sIUD group had the youngest gestational age at birth and that the 14-19 weeks at sIUD group had the oldest gestational age at birth. Also, we found that the prolongation of gestational age in the surviving twin was shortened as the gestational age at sIUD increased.
Gestational age, whether for singletons or twins, is a key factor in determining the prognosis of surviving fetuses, since the risk of adverse consequences declines with increasing gestational age.12 Therefore, we focused on the gestational age at sIUD and at birth to evaluate the prognosis of MC twins after sIUD. After analysis, we found that older gestational age at sIUD is associated with PTB in MC twins. When we further divided these cases into four groups according to their gestational age at sIUD, we found that the 25-30 weeks at sIUD group had the youngest gestational age at birth and the 14-19 weeks at sIUD group had the oldest gestational age at birth in all the cases. including spontaneous preterm birth cases. This indicates that MC twins undergoing sIUD at 14-19 weeks have the best prognosis out of these groups and that MC twins that undergo sIUD at 25-30 weeks have the worst prognosis.
In figure 1 and figure 3, we found that the first three groups (14-19 weeks, 20-24 weeks, and 25-30 weeks) showed a decreasing trend in their association with the prolongation of gestational age of the surviving twin. However, the fourth group (31-36 weeks) was not in line with this expected negative trend. This discrepancy was due to the fact that even if the fourth group cases immediately delivered after sIUD, the numerical gestational age at birth was larger than those of other groups. Therefore, we introduced the concept of the prolongation in gestational age after sIUD as the evaluation factor to avoid the impact of the numerically large gestational weeks in the third trimester. Through this new factor, we found that increases in the gestational age at sIUD led to a shorter prolongation in gestation of the surviving twin. We know that in late pregnancy, the area of placenta is much bigger than during early pregnancy. Along with this fact, past studies have reported that placental ischemia can release various factors leading to uterine contraction. 13 This suggests that the impact on the uterus caused by a decrease in placental circulation is much bigger in later gestational ages. Therefore, the shorter prolongation in gestation of the surviving twin may be due to the effective circulating blood volume of the placenta decreasing sharply when sIUD occurs, leading to instantaneous placental ischemia-induced uterine contractions. Because the area of placenta in early pregnancy is much smaller than that in late pregnancy, the stimulation of uterine contractions caused by the decrease in placental circulation is accordingly smaller. This may explain why increases in the gestational age at sIUD leads to a shorter prolongation of gestation in the surviving twin.
A previous systematic review assessing the co-twin outcome after sIUD reported that the incidence of PTB after sIUD was 68% in MC twin pregnancies.7 Another study reported the incidence of PTB after sIUD as being up to 66% in MC twin pregnancies.14 However, in our study, the incidence of PTB was much higher (89.7%) than these previous studies, while the incidence of spontaneous PTB was similar to other studies. This discrepancy may be due the higher rate of iatrogenic premature delivery at our hospital, which is a referral center responsible for difficult and critical cases in the three northeastern provinces. In addition, most of the cases of iatrogenic PTB were accompanied by maternal complications and fetal complications, which led to complex and rapid changes making iatrogenic preterm delivery inevitable.
This study is not without its limitations. Due to the retrospective design and lack of consistent documentation, potential residual confounders, such as obesity, smoking, and living environment factors, could not be properly evaluated. Autopsy and placental evaluations may also have been limited, particularly if the prolongation of gestational age was long after sIUD. On the other hand, our study offers a novel insight into the association between the gestational age at sIUD and the subsequent prolongation of gestation in the surviving twin. This insight can help clinicians to better stratify the prognoses in women experiencing sIUD in MC twin pregnancy to better support the surviving twin.