Implications (in light of other evidence)
Our finding regarding the increased risk for subsequent stillbirth in
pregnancies with a previous CD is consistent with several other
studies18, 24-28. However, most of the previous
studies had inconsistencies and weaknesses in the methods and
statistical analysis. For example, most studies did not exclude
stillbirth from the first birth which is strongly associated with
subsequent stillbirth21.
Our findings agree with those
from a recent Norwegian cohort study of 294 598 singleton second births,
which reported a 33% increased odds of antepartum stillbirth (aOR:1.33
[95% CI, 1.08–1.63)] and a 84% increased odds of intrapartum
stillbirth (aOR:1.84 [95% CI, 1.00–3.38])18,
although the timing of CD was not considered in this study.
When analysed by cause of fetal death, excess risk was apparent of
unexplained stillbirth from 34 weeks’ gestation as previously
observed10, 26. It should be noted however that both
studies failed to adjust for maternal comorbidities and our results
attenuated to almost no effect in the adjusted model.
We found no increased odds of intrapartum stillbirth in women who had
VBAC compared to those with repeat CD. It must be acknowledged however
that there were few cases of intrapartum stillbirth (n=13) in women who
had a VBAC. To our knowledge, none of the previous studies assessed the
risk of stillbirth while adjusting for potential confounders in women
who had a VBAC, possibly due to the rarity of this event. Findings from
a systematic review and meta-analysis suggested a higher rate of
perinatal mortality in women who had VBAC compared with a planned repeat
CD (0.13% versus 0.05%), but no data were reported on
stillbirth19.
Two studies reported inconsistent results for the association between
perinatal death and neonatal death in women with a first CD undergoing
VBAC29, 30. Smith et al. assessed the risk of
perinatal mortality, including intrapartum stillbirth and neonatal death
using a large Scottish registry data of 313,238 singleton births and
reported 11 times greater odds (aOR:11.7 [95% CI, 1.4–101.6])
compared to women with a repeat CD29. However, the
reported CI was very wide, and the authors did not adjust for maternal
comorbidities. The second study by O’Neill et
al .30 evaluated the risk of neonatal and infant death
in women with a VBAC compared to women with a repeat CD using data from
the Danish registry, including 61,626 births. The authors reported no
increased odds of late neonatal death (aOR:0.97 [95% CI, 0.22–4.32)
or infant death (aOR:1.12 [95% CI, 0.79
–1.59])30.
Our analysis demonstrates that maternal conditions (including
hypertensive disorders of pregnancy) and intrapartum asphyxia have a
significant impact on the association between CD and subsequent
stillbirth. This provides information that could formulate a causal
hypothesis for the observed association. A case control study found that
the uterine artery Doppler waveform between 18-22 weeks’ gestation is
more likely to be notched (14.5% vs. 6%, p<0.001) and have a
higher average pulsatility index (1.25 vs 1.16, p<0.02) in
women who had an elective CD compared to a VB31.
Notably, women who had a prior CD also have a higher incidence of
preeclampsia32, 33. Further research is required to
further understanding of a causal relationship between CD and
stillbirth; these studies suggest that investigation of impaired
placental blood flow merits further exploration.
In case of antepartum stillbirth,
it is common practice and a safe option for women with a previous CD to
have VBAC as most of the risk of VBAC relates to the
fetus34. Both the Royal College of Obstetricians and
Gynaecologists and the American College of Obstetricians and
Gynecologists encourage that VB should be offered over CD in case of
antepartum stillbirth.34 This could explain the
observed association between VBAC and antepartum stillbirth.