It's largely to do with how long the doctor has been at work for
With one in 7000 mothers dying in childbirth each year and 15 babies a day either being stillborn or dying within a month of birth, infant and maternal mortality rates in the UK still need dramatic improvement.
Childbirth is notoriously painful and can be extremely dangerous.
As to why this the case for humans compared to other mammals is still widely debated and researched.
As for IF there's a particularly dangerous time for mum and baby, this may have been answered.
The most dangerous time to give birth.
There have been numerous studies into this subject and a range of results, but now scientists at the University of Texas at Austin have come up with a new theory.
The results are linked to doctor's shift
They claim the number of hours an obstetrician has been at work prior to an unscheduled delivery is what "significantly influences" risks to both mum and baby.
When a doctor enters the ninth hour of a 12 hour shift, the researchers found, there is an increased risk of a mum losing a dangerous amount of blood or a newborn suffering low oxygen levels.
Dr James Scott, an associate professor of statistics, said: "There are all sorts of studies about the timing of deliveries, but what nobody had looked at before is whether there is some kind of proxy for how fatigued the doctors are.
There's been a lot of debate on the topic of infant and maternal mortality (Photo: Getty)
"We find there's a peak eight to 10 hours after the beginning of a shift when, relative to baseline, the risk of maternal blood loss exceeding 1.5 litres increase by 30 per cent, and arterial pH, a marker for infant distress, is at increased risk of falling below 7.1."
Normal arterial pH is should be between 7.3 and 7.4.
The team behind the research identified how fatigue will contribute to a doctor missing small foetal distress signals.
These can be detected if a doctor is alert and less tired.
Doctors were studied for five years
Researchers looked at 24,506 unscheduled deliveries in the UK between January 2008 and October 2013.
All the obstetricians observed were from the same maternity wards and worked 12-hour shifts throughout the study.
There was no difference between day deliveries and ones which happened at night.
Nor for weekday versus weekend deliveries, vaginal or C-section deliveries or whether a doctor was a junior or more senior.
But the ninth hour of a shift was demonstrably more dangerous.
Surprisingly, the last two hours showed a decrease in danger, which is thought to be attributed to the doctors on duty passing on more complicated cases to their colleagues starting their shift
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