A quarter of all babies in England are now delivered by emergency caesarean operations, BBC analysis shows - marking a significant rise over the last five years.
The unplanned surgeries have increased by eight percentage points, while the rate of elective caesareans has also increased.
At the same time, the rate of vaginal births without instruments has fallen - from more than half of all deliveries to 43%.
Prof Marian Knight, director of the National Perinatal Epidemiology Unit, which researches the care of women and babies in pregnancy and birth, says the rise represents a "total change in how women give birth" in England, and that it has not been replicated in other European countries.
The NHS does not publish data on why an emergency C-section is performed, and experts say there is no single, clear explanation for the increase.
However, some have told the BBC they are concerned a culture of fear in maternity units and among pregnant women is driving up the number of procedures.
The Royal College of Obstetricians and Gynaecologists, which represents maternity doctors, says pressure on staff and operating theatres means the system is "really struggling" to meet the increased demand.
NHS England says "decisions are made by considering individual circumstances and clinical advice to ensure the safest and most appropriate approach for each birth".
A caesarean section, or C-section, is a cut through the mother's tummy and womb.
Emergency C-sections are graded from the most urgent - where there is an immediate threat to the life of the woman or the baby - to those where labour is not progressing well.
BBC Verify has tracked the changes in births in England over five years.
The most common way to have a baby is still a vaginal delivery where medics don't use instruments such as forceps to help pull the baby out - but that has fallen from 53% to 43% of births.
Planned caesareans now make up 20% of births and there has been a steady increase in emergency caesareans - from 18% to 26%.
Figures from other parts of the UK are not as up to date, but put emergency caesarean rates at 22% in Scotland, 20% in Wales and 16% in Northern Ireland.
Prof Knight's unit has compared how caesarean birth rates - both planned and emergency - have changed across 42 countries.
Her team ranked each country on the proportion of C-section births, from highest to lowest. In 2020, England was 14th out of 42 countries - but by 2025, it was up to 9th.
She says the other countries are generally not seeing such sharp increases.
Despite the rise in emergency C-sections since 2020, stillbirths and neonatal mortality have remained largely unchanged.
Shakila Thangaratinam, professor of women's health at the University of Liverpool and a consultant obstetrician, says: "An increase in emergency caesarean sections poses concern if these are not accompanied by a corresponding decrease in stillbirths, newborn and maternal complications."
She says reasons for the rise are largely unknown, adding there is "no clear and consistent reporting". And she says it will not be possible to explain increases without "good quality data" about why operations are performed.
Prof Thangaratinam says she also wants to explore what part race has to play. While the national average for emergency C-sections is one in four births, it is about one in three for black and Asian mothers.
Prof Knight is investigating whether factors like age, obesity and pre-existing medical conditions are contributing to the increase - but she says NHS staff and mothers may also be influenced by high-profile maternity scandals.
For years, maternity units were told to keep rates of caesareans low. Those targets were dropped in 2022.
Recent maternity safety scandals in Morecambe Bay, East Kent and Shrewsbury and Telford all heard similar, devastating stories of mothers and babies dying and a reluctance to do caesareans. Ongoing inquiries in Nottingham and Leeds are likely to have similar concerns.
"We also need to recognise the potential impact of rising fear among women, families and staff, which may lead more to choose or to recommend caesarean birth," she says.
Another health professional told the BBC no one wants to be the next scandal.
Over the last five years, there has also been an 11% increase in the number of legal claims against the NHS for maternity problems.
"Legal cases typically question why caesareans are not undertaken or not undertaken sooner," Prof Knight says. "Doctors and midwives are rarely criticised for performing an early caesarean."
NHS England says "many factors" influence caesarean births and its "priority is always the safety and wellbeing of both mothers and babies".
The BBC spent two days in the maternity unit at Northwick Park Hospital in London and met 18-year-old new mum Khushi.
She had an emergency caesarean because her baby's heart rate kept slowing down during labour. It was classed as category one, the most urgent type.
Khushi says she was baffled and scared - that she had no clue what she was getting herself into.
"It was my first ever surgery. Just the thought of being open on a table still feels so surreal and so unbelievable," she adds.
Now at home with baby Aarav, she faces six weeks physical recovery from the major surgery, but says the mental trauma is the hardest part for her.
Dr Alison Wright, president of the Royal College of Obstetricians and Gynaecologists, has been delivering babies for 35 years. She worries how services will adapt to the increased number of emergency C-sections.
She says many maternity units already don't have enough dedicated obstetric theatres.
"If we do not invest in our workforce and in our operating theatre capacity, we may be in a position in the future where we cannot do the emergency caesareans we need to," she adds.
Prof Ed Wilson, health economist at the University of Exeter, says that according to NHS data, a routine vaginal delivery is almost £4,800. A planned caesarean would be about £6,000.
But he estimates that emergency C-sections cost almost £9,000.
An NHS spokesperson says: "The increase in the number of emergency caesarean births is influenced by many factors, and our priority is always the safety and wellbeing of both mothers and babies.
"Decisions are made by considering individual circumstances and clinical advice to ensure the safest and most appropriate approach for each birth."
The Department of Health and Social Care says it was committed to improving maternity and neonatal safety and pointed to Health Secretary James Murray's role as chair of the national maternity taskforce.
