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A late-labour stage Caesarean section, especially when performed at full dilatation (cervix 10cm open), increases the risk of subsequent spontaneous preterm birth (<37 weeks). In the next pregnancy, women who have had a Caesarean section at full dilatation have a 15% chance of delivering spontaneously preterm, higher than women who had a vaginal birth (7-8% preterm birth rate) or an early-labour stage Caesarean section (2-3% preterm birth rate).
The cause is thought to be damage to the cervix and lower uterine wall related to the low position of the Caesarean section scar when the labour is advanced (Figure 5). We have worked with the team at St Thomas’s Hospital London as part of the CRAFT study to investigate why this is, and how we can prevent preterm birth.
Using a novel ultrasound technique developed at UCL we have demonstrated that Caesarean section scar position and characteristics are important predictors of spontaneous preterm birth in the next pregnancy. The internal cervical os (top of the cervix) is identified using colour Doppler at the level of the uterine arteries using colour Doppler ultrasound (Banerjee et al. 2022). The scar distance is then measured.
We have recently shown that a low Caesarean section scar – one situated within 5mm of the internal os or within the cervix itself – leads to a shortening if the cervix and preterm birth. It is reassuring that early placement of a cervical cerclage appears to be effective in reducing the preterm birth rate. At UCLH we specialise in the care of women with Caesarean scar damage in our Preterm Birth Clinic. We are now studying how Caesarean section scar position and characteristics change after birth and into the next pregnancy.
Figure 5. Ultrasound image of the cervix which has started to funnel from the top. A Caesarean section scar (white arrows) can be seen near the top of the cervix.