Results

During the study period (1982-2012), 1,771,700 singleton births from 885,850 women were identified as eligible participants. Of the 885,850 women in the cohort, 2,428 had stillbirth in the second pregnancy (2,292 were antepartum stillbirths and 136 were intrapartum stillbirths), resulting a rate of 2.7 per 1,000 births. Women who had stillbirth in second pregnancy were older, more likely to be smokers, had higher BMI and had higher rates of chronic hypertension and diabetes, but lower gestational age compared with women who had live birth in their second pregnancy (Table 1).
The ReCoDe classification system indicated that fetal causes accounted for approximately 27% of the total stillbirths and the SGA\fetal growth restriction was the most frequent factor (21.2%, Table 2). The second most frequent cause was placental abruption (5.5%), followed by intrapartum asphyxia (4.4%).
The demographic characteristics of women and their babies according to mode of delivery are shown in Tables S3-S5. In first pregnancy, 117,114 (13.2%) mothers had a CD, while 768,736 (86.7%) had a VB. In the second pregnancy, almost half (53.6%) of the women with a previous CD also had CD in their subsequent pregnancy compared with 46.3% women who had VBAC. While 5.4% of mothers who had VB in first pregnancy had CD in their subsequent pregnancy.
Table 3 presents the crude and adjusted ORs of the association between CD in the first pregnancy and the risk of subsequent stillbirth. After adjusting for potential confounders, mothers with a previous CD had higher odds for antepartum stillbirth (aOR:1.35 [95% CI, 1.21–1.51]), intrapartum stillbirth (aOR:1.67 [95% CI, 1.09–2.53]), and any stillbirth (aOR:1.37 [95% CI, 1.23–1.52]) compared with mothers with a previous VB.
Analyses by type of CD in the first pregnancy showed increased odds of any subsequent stillbirth in women with a pre-labour CD (aOR:1.31 [95% CI, 1.09–1.58]) and in-labour CD (aOR:1.36 [95% CI, 1.19–1.55]), compared to women with a previous VB (Table 4). The odds of antepartum stillbirth was similarly higher in mothers with a previous pre-labour CD (aOR:1.24 [95% CI, 1.02–1.50]) and in-labour CD (aOR:1.36 [95% CI, 1.19–1.55]), compared to mothers with a previous VB. However, the risk of intrapartum stillbirth was higher in pre-labour CD group (aOR:2.72 [95% CI, 1.51–4.91]) than the in-labour CD group (aOR:1.35 [95% CI, 0.76–2.40,]), although not statistically significant for the latter group. Additionally, there was no statistically significant association between subsequent stillbirth and prior instrumental VB (Table 4).
Compared to women with a repeat CD, women with VBAC had an increased odds of antepartum stillbirth (aOR:4.49 [95% CI, 3.55–5.67]), but no association was found for intrapartum stillbirth (aOR:0.99 [95% CI, 0.48–2.06]) (Table S6). Similar results were found when VBAC was grouped according to type of CD in the second pregnancy into: (1) VB after pre-labour CD and (2) VB after in-labour CD. Both types of CD were associated with a greater odds of antepartum stillbirth but not intrapartum stillbirth (Table S10).
On the other hand, women who had CD after VB had an increased odds for both antepartum stillbirth (aOR:2.50 [95% CI, 1.92–3.25]) and intrapartum stillbirth (aOR:3.01 [95% CI, 1.67–5.43]) compared to women who had a repeat CD (Table S6). However, in the subgroup analyses by types of CD, the increased odds of antepartum and intrapartum stillbirth was only observed in women who had in-labour CD after VB (aOR:3.67 [95% CI, 2.76–4.89] and 5.86 [95% CI, 3.20–10.7], respectively, Table S10) suggesting that increased risk of stillbirth could be due to complications during birth.
The results from the subgroup analysis by cause of stillbirth according to ReCoDe classification (Table S7) suggested that maternal conditions (aOR:1.78 [95% CI, 1.31–2.42]) and intrapartum asphyxia (aOR:2.04 [95% CI, 1.41–2.97]) have a significant impact on the association between CD and subsequent stillbirth. However, the results of other causes did not reach a statistically significant level (Table S7). Additionally, we found a 69% increased odds of explained stillbirth, with any known relevant condition (OR:1.69 [95% CI, 1.46–1.94]), but almost no effect for the unexplained cases (OR:1.08 [95% CI, 0.92–1.27]). We also found that the risk of unexplained stillbirth differed with gestational age. In an additional analysis of restricted gestational age ≥34 weeks (N= 872,351), the odds of unexplained stillbirth was 1.18 [95% CI, 1.00–1.38]), but this has attenuated after adjusting for confounding factors to 1.11 [95% CI, 0.94–1.30].
Results from sensitivity analyses were similar to the main findings (Appendix pages 1-2, Tables S8-S9). The population AF associated with prior CD was 0.049, meaning that CD in the first pregnancy accounted for approximately 5% of all subsequent stillbirths in the studied population.