<div>Mini-commentary on BJOG-20-1459.R1: Caesarean birth and risk of
subsequent preterm birth: retrospective cohort study</div><div>Declarative title to be added</div><div>Bradley de Vries</div><div>School of Public Health</div><div>University of Sydney</div><div>Sydney</div><div>New South Wales</div><div>Australia</div><div>Worldwide, preterm birth occurs in 11% of pregnancies and is the
leading cause of childhood mortality. Complications from preterm birth
are the most common cause of neonatal death in the United Kingdom, yet
the incidence of preterm birth is not falling.</div><div>In this issue of BJOG, Williams <i>et al</i> (BJOG xxxx) report on an
observational study which identifies previous second stage caesarean
birth as a risk factor for spontaneous preterm birth, confirming the
results of other observational cohorts. Among women with one previous
term birth, the adjusted odds ratio was 2.1 (95% confidence interval
[CI] 1.3 to 3.1) for preterm birth before 37 weeks gestational age
and 7.5 (95% CI 3.4 to 15) for preterm birth before 34 weeks, for
previous second stage caesarean birth compared with previous vaginal
birth. They adjusted for confounders not addressed in other studies
including interpregnancy interval and maternal deprivation index,
strengthening the existing evidence. The association is plausible
because the cervix and lower uterine segment are anatomically merged in
the second stage of labour and inadvertent cervical incision might
damage the integrity of the cervix. The association was at least as
strong as that described for previous excisional surgery for cervical
dysplasia. Current National Institute of Clinical Excellence Guidelines
recommend considering prophylactic cervical cerclage for an
ultrasound-measured cervical length &lt; 25mm if there is a
history of cervical trauma. Given the plausibility and emerging
epidemiological evidence, it would seem prudent to offer the same
screening and treatment when there is a history of second stage
caesarean birth.</div><div>The observed association is relevant in other ways. Counselling about
instrumental versus caesarean birth may be influenced by knowledge of
future risks. Additionally, surgeons may need to be aware of the
potential importance of avoiding inadvertent cervical incision, while
still avoiding upper segment incision and its attendant risk of future
intrapartum uterine rupture.</div><div>The study by Williams <i>et al</i> is well designed, yet there remains
potential for confounding not adjusted for in the analysis. Further,
missing data for body mass index (1.6%) and cigarette smoking (13%)
were classified as unknown and not imputed which can also cause bias. As
there are only a handful of observational studies, it would be sensible
to confirm the association, and explore potential causative mechanisms
(e.g., by monitoring cervical length in subsequent pregnancies).</div><div>Clinical prediction models for spontaneous preterm labour in
asymptomatic women have been developed but need improvement before
incorporation into clinical practice. Addition of new risk factors such
as second stage caesarean birth and better understanding of the causes
of preterm birth could improve these models and ultimately improve
outcomes through offering prophylaxis with cervical cerclage, vaginal
progesterone, or pessary for women at high risk.</div><div>Given the massive personal, clinical, and economic burden imposed by
preterm birth, the plausibility of the association, and the growing
evidence from observational studies, I believe cervical surveillance
warranted, with a view to offering prophylactic measures when there is a
history of second stage caesarean birth.</div><div><b>No disclosures:</b> A completed disclosure of interest form is
available to view online as supporting information.</div>