Restricted growth in unborn children is the single largest risk factor for stillbirth, research has found.
It brings especially high risk when it goes unrecognised before birth, and spotting foetal growth problems early could cut UK stillbirths by 600 a year, the study published in online journal BMJ suggested.
Detection before birth must become a cornerstone of antenatal care, its authors said.
The UK has one of the highest stillbirth rates in the developed world.
Often considered unexplained and unavoidable, rates have changed little over the last two decades.
Recently, doctors have found that many stillborn babies fail to reach their growth potential, prompting a renewed focus on what causes foetal growth restriction.
In response to this a team of researchers at the West Midlands Perinatal Institute in Birmingham set out to assess the main risk factors associated with stillbirth in a multi-ethnic population.
Using NHS records, they identified 92,218 normally formed single babies, including 389 stillbirths, from 24 weeks of gestation, delivered during 2009-11 (a stillbirth rate of 4.2 per 1,000 births).
They then assessed several maternal and foetal risk factors for stillbirth and calculated the proportion of stillbirths that could be potentially avoided if these risks were removed.
These included mother's age, parity (the number of times she had given birth), body mass index, mental health history, pre-existing diabetes, high blood pressure or heart disease, smoking in pregnancy, alcohol consumption and foetal growth problems.
First, third and subsequent pregnancies were associated with an increased risk of stillbirth compared with second pregnancies, but high maternal age carried no increased risk.
Ethnicity (African, African-Caribbean, Indian and Pakistani) carried a higher risk, as did deprivation and unemployment of the mother or her partner.
The research found that maternal obesity (women with a BMI of 30 or more), smoking, pre-existing diabetes, a history of mental health problems and foetal growth restriction were all associated with a significantly increased risk.
As potentially modifiable risk factors, maternal obesity, smoking in pregnancy and foetal growth restriction together accounted for 56% of all stillbirths.
But the strongest risk factor was foetal growth restriction, which carried a four-fold higher risk of stillbirth compared with normal growth pregnancies.
This increased to an eight-fold risk if it was not detected during pregnancy, accounting for 32% of all stillbirths in the study.
The authors point out that, despite this, the presence of foetal growth restriction is currently missed in most pregnancies and they estimate that 71 stillbirths in their study population could have been avoided through better antenatal recognition.
If applied to the UK population, this would represent 600 fewer stillbirths per year.
A spokesman for the authors said: "Our study shows that while there are several risk factors for stillbirth that can be ascertained from the outset of pregnancy, the single largest factor is foetal growth restriction, which is currently not well predicted and not recognised antenatally in most pregnancies.
"Most normally formed singleton stillbirths are potentially avoidable and preventive strategies need to focus on improving antenatal detection."
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Charlotte Bevan, research and prevention adviser at stillbirth and neonatal death charity Sands, said it welcomed the findings.
She said: "Seventeen babies die every day in the UK before, during and shortly after birth.
"Many of these mums are in low-risk pregnancies close to or at term and the death of their baby comes as a devastating shock.
"To discover subsequently that their baby was not low-risk at all but was growth restricted and that this is a well-known risk factor for stillbirth leaves families bewildered and struggling to come to terms with their grief.
"This research shows that routine antenatal care fails hundreds of families every year and action to improve care is desperately needed."