Women who have Caesarean births at an advanced stage of labour are about eight times more likely to develop scars in the womb which are known to increase the likelihood of premature births in future pregnancies, UCL researchers have found.
The study, published in the American Journal of Obstetrics & Gynecology, looked at how the stage of labour when the operation is performed affects where the scar forms and how well it heals. More than 40 per cent of all births in high-income countries including England are now by Caesarean.
As labour progresses, the baby moves further down the woman's womb towards the vagina. In preparation for giving birth, the woman's cervix – the opening of the womb – opens or 'dilates' up to 10 centimetres.
In some cases, doctors may need to intervene and perform a Caesarean procedure for safety reasons. It is a major operation and leaves a scar in the womb.
An earlier study by the same team of UCL researchers found that when this internal scar was close to the cervix, the risk of the woman having a preterm birth (before 37 weeks) in future pregnancies was significantly increased.
In their new study, they wanted to better understand the factors that influence where in the womb these scars are located by scanning women after a Caesarean birth.
They found that women who had a Caesarean at an advanced stage of labour (when the cervix was almost fully (around 8cm) or completely open) were about eight times more likely to have scars that formed within or near the cervix. In addition, a low position of the baby in the womb increased the chance of a low scar.
The researchers also found that scars lower down the womb healed less well than those higher up the womb.
They say their findings will help better plan follow-up care for women in future and ways for them to avoid preterm births with future pregnancies. The results should also allow doctors to explore ways to improve surgical techniques to reduce risks in the future, they add.
They are calling for more research into the impact of Caesarean scars on gynaecologic symptoms and future pregnancies and to help develop techniques to improve Caesarean-scar healing.
Lead author Dr Maria Ivan (UCL EGA Institute for Women's Health) said: "We knew that having a Caesarean birth can damage the cervix. Our study is the first to look at where that damage in the womb is depending on how advanced the labour is when the operation takes place.
"Our findings have significant implications for women who have a Caesarean late in their labour and want to have more children.
"This is particularly relevant given the huge increase in the number of women who are having Caesarean births over the last decade.
"These fresh insights can help shape the future care for women having advanced labour Caesarean births, helping doctors better prepare women for giving birth and hopefully also leading to improved surgical techniques which reduce the risk of future complications that Caesarean delivery scars can cause."
Ninety-three women who had a Caesarean delivery during active labour (defined as the cervix being at least four centimetres dilated) took part in the study.
The researchers examined them using transvaginal ultrasound between four and 12 months after birth to see if the Caesarean operation had left a scar and if so, where it was in the womb.
Almost all of the 93 women – 90 – were found to have a visible internal scar.
The researchers found that for each one-centimetre increase in cervical dilatation during labour, the scar was positioned 0.88mm lower down the womb and closer to the cervix.
Fifty-two of the women's scars (57.8 per cent) were in the higher part of the womb, with the scars in the rest of the women located either close to the cervix (19 women or 21.1 per cent) or within it (also 19 women or 21.1per cent).
From this, the researchers worked out that having an advanced labour Caesarean birth was associated with an eightfold higher risk of the scar being located near or within the cervix compared to Caesarean births at an earlier stage of labour.
The researchers also looked at the impact the position of the scar had on how well it healed.
Indicators of impaired scar healing included the presence of a scar "niche" – a gap or defect in the wall of the womb at least two millimetres deep. These niches form a pouch where blood can accumulate and may be associated with infertility, irregular menstrual bleeding or complications in future pregnancies.
Based on these indicators, they found that scars which formed close to or within the cervix did not heal as well as scars higher up the womb, posing a greater risk of complications in future pregnancies.
Co-author Professor Anna David, EGA Institute for Women's Health,
National Institute for Health Research University College London Hospitals Biomedical Research Centre and Deputy Director of charity Tommy's National Centre for Preterm Birth Research, said: "Caesarean birth in advanced labour is known to be linked with preterm birth. Our findings highlight the importance of healing of the Caesarean scar in the womb and that the stage of labour and low position of the baby can impact how this happens."
Dr Jyotsna Vohra, Director of Research, Programmes and Impact at Tommy's, the pregnancy and baby charity, said: "Unplanned Caesarean births can be very stressful, especially when carried out in an emergency to make sure baby arrives safely. Women who have experienced this may already be feeling anxious going into their next pregnancy, and to be then told they're at increased risk of giving birth early will only add to the anxiety.
"This new research paves the way for better prediction and prevention of preterm birth following a previous Caesarean birth. We're working closely with healthcare professionals to make sure this research breakthrough translates to improvements in care for women and birthing people whose babies are most at risk of being born too soon."
The number of women having Caesarean births has risen sharply in England in recent years to 42% (225,762, of which 99,783 were elective and 125,979 registered as emergency Caesareans) of all deliveries in 2023/24 compared with 26% (166,081 deliveries) in 2013/14. ¹
