Methods
This retrospective cohort study was conducted at West China Second University Hospital (Sichuan University), a tertiary care hospital. Women with a history of CS who gave birth from January 2016 to December 2016 at West China Second University Hospital were included in this cohort. Data were obtained from the electronic medical system and were supplemented by information from medical records. Two trained abstractors were instructed to review and abstract data from the medical chart. The ethics committee and data inspectorate of West China Second University Hospital of Sichuan University approved this study (ref. number 2016-010; date of approval 2016-05-25). We conducted this observative study following the STROBE guideline.
Our primary analyses included all women who had at least one previous CS at 28 to <42 weeks of gestation and delivered at 28 to <42 weeks of gestation. We excluded women who met the following criteria: (1) underwent uterine surgery other than CS; (2) did not receive routine antenatal care; (3) had multiple pregnancies; and (4) had fetal malformations.
IPI was defined as the number of months between the date of last cesarean operation and the conception date of the subsequent delivery, which was estimated by the delivery date of the index pregnancy minus its gestational age at birth. The IPI was categorized into four groups: less than 24 months, 24 to 59 months, 60 to 119 months, and 120 months or greater.
Maternal outcomes included anemia in late pregnancy, GHP, GDM, an abnormal placental position, pernicious placenta, placenta accreta, placental abruption, premature rupture of membranes (PROM), uterine rupture (including incomplete and complete uterine rupture), postpartum hemorrhage (PPH), and hysterectomy. According to the WHO guidelines, anemia in late pregnancy was defined as a hemoglobin concentration less than 110 g/L between 32 and 42 weeks of gestation.17The definitions of gestational hypertension complied with the consensus statement from the International Society of Study of Hypertension in Pregnancy (ISSHP).18 The definitions of GDM (where Grade A1 indicates that blood glucose is well controlled by nondrug therapy during pregnancy and Grade A2 indicates that blood glucose should be controlled by drugs during pregnancy) complied with the WHO criteria.19 Uterine rupture was measured during CS by experienced obstetricians, and complete rupture was defined as a tear through all layers of the uterine wall, and incomplete rupture was defined as a tear in the muscular layers with intact serosa or amniotic membranes.20 Adverse fetal outcomes included preterm birth, admission to the NICU, low 1-min Apgar score, and LBW. LBW was defined as a birth weight less than 2500 g, and a low 1-min Apgar score was defined as an Apgar score of less than 10 during 1 minute after birth evaluated by a professional obstetrician.
The demographic and clinical characteristics of the four groups may be significantly different, and these factors may be potential confounding factors affecting the association between IPI and perinatal outcomes. We established multivariable logistic regression analysis models to adjust for these factors and then analyzed the effect of the IPI after CS on adverse maternal and fetal outcomes. Since previous studies have confirmed that emergency CS significantly increases the risk of adverse perinatal outcomes,21 we established analysis models to separately analyze the relationships between IPI after emergency CS or elective CS and pregnancy outcomes by using a multivariable logistic regression method.
Sociodemographic characteristics and adverse pregnancy outcomes were compared among the four groups, the chi-squared test was used for categorical variables, and one-way analysis of variance was used for continuous variables. Multivariable logistic regression analysis was separately conducted for the association between different IPIs and each pregnancy outcome after controlling for the known and suspected confounding factors, and the analysis used an IPI of 24-59 months as the reference category, as it was the recommended IPI by WHO guidelines.1 Data analyses were performed using the software package SPSS statistics version 21.0 (SPSS Inc.) A p-value < 0.05 for both sides was considered significant.