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.