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.