Interpretation
Because birth spacing might affect maternal health status and nutrient
concentrations, it is plausible that the IPI after CS is associated with
perinatal outcomes. Some studies have recognized the increased risk of
anemia in women with IPIs shorter than 6 to 24 months regardless of a
history of CS.23-25 A study found that a short IPI of
<18 months after CS was related to a greater risk of
anemia.26 Our study indicates that an IPI of
<24 months increased the risk of anemia in late pregnancy.
Anemia among women with a short IPI is a status of total iron depletion,
which is seen to be the consequence of inadequate time from previous
delivery to replenish the iron stores.27
Many studies have reported the association between birth spacing and
adverse neonatal outcomes,15 while limited studies
have been conducted exclusively for women with a history of a previous
CS. A large meta-analysis of multiparous women in 2006 concluded that
IPIs of <18 months and of ≥ 60 months had the highest risk for
preterm birth.15 Moreover, Class et al used cousin and
sibling comparisons and revealed that an IPI of 60 months or greater had
elevated risks for preterm delivery (< 37
weeks).28 Compared with an IPI of 24-59 months, our
results implied that the rate of premature delivery before 34 weeks
increased in the group of women with an IPI of <24 months and
of ≥120 months. Due to the substantial growth of parturients with
extremely long IPIs after implementation of the revised reproductive
policy in China, we separately analyzed the association between adverse
perinatal outcomes and IPIs of ≥120 months, which was not addressed in
the non-Chinese studies. Zhu et al attributed the relationship between
long intervals and adverse perinatal outcomes to “physiological
regression”, the hypothesis being that women’s reproductive capacity
after delivery physiologically declines and gradually reaches the levels
of primigravid women.29 The reason for the lack of an
association between a short IPI after previous CS and adverse perinatal
outcomes could be the small proportion (7.28%) of short IPIs in this
study.
The relationship between IPI after CS and abnormal placental position
was reported in two studies, which reported that both were
unrelated.30,31 Our investigation also indicated no
association between abnormal placental position and IPI. The association
between a short IPI and an increased risk of inadequate healing of
uterine scars has been recognized.32 Although our
analysis included maternal age as a potential confounder, we still
attribute the association between a long IPI and abnormal placental
position to the physiological changes in the uterus in AMA. Several
studies have observed an increased occurrence of placenta previa among
women with AMA and thought that AMA may lead to compromised
uteroplacental blood flow, thus increasing the risk of placental
previa.33-35 We found that since only 9 patients were
pregnant within 1 year after CS and none of them had abnormal placental
positions, there may be a bias.
Our investigation demonstrated that the incidence of GHP and GDM was
significantly increased among women with IPIs of ≥120 months.
Nevertheless, the effect was nonsignificant after adjustment. Hanley et
al showed that women with a longer IPI were more likely to develop GDM
or GHP,36 but the study took women with multiple
pregnancies as its own control, and maternal age increased with
increasing in IPI. In our study, women with an IPI of ≥120 months were
significantly older than those in the other three groups, and nearly
80% of these women had a gestational age of over 35 weeks. In addition,
overweight and obesity have also been proven to be risk factors for GHP
and GDM.23,24 However, in our study, there were no
significant differences in the proportions of high maternal BMI among
the four groups. Therefore, the correlation between GHP, GDM and IPI may
be affected by maternal aging.
Repeat CS is acknowledged to increase the risk of maternal
complications,37 and TOLAC is an option for women with
a scarred uterus. Successful TOLAC can avoid numerous surgery-related
morbidities and is beneficial to subsequent pregnancies. However, failed
TOLAC is related to an increased risk of uterine rupture, which is
detrimental for both the parturient and the
fetus.38,39 A systematic study reported that the
median incidence of complete uterine rupture was 1% among women with
previous CS,40 but the range of incidences of
incomplete uterine rupture was wide at
2.1%-10.1%.41,42 In this study, 8.7% (n = 145) of
women were found to have incomplete uterine rupture during the
operation, while no complete uterine rupture was reported. Previous
studies indicated that the risk of complete uterine rupture increased
with AMA, grandmultiparity (≥ 6), an extremely short IPI and
macrosomia.6,43-45 We found that the IPI of women with
a history of emergency CS was not related to the occurrence of uterine
rupture, while in the group of women with a history of selected CS, the
risk of incomplete uterine rupture decreased in the groups with IPIs of
60-119 months and ≥120 months. Studies have reported a significantly
increased risk of uterine rupture during emergency CS, but mothers with
elective repeat cesarean section (ERCD) had almost no risk of uterine
rupture (0-0.004%).44 We assumed that the risk of
uterine rupture was underestimated for elective CS, mitigating the risk
of uterine rupture.