Pregnant women have a new test for a deadly complication which affects 50,000 people a year.
A simple urine test could detect pre-eclampsia up to 10 weeks earlier, according to doctors in the US.
Currently, the complication affecting one in 12 pregnancies first comes to light at antenatal appointments.
High blood pressure and protein in the urine are the first warning signs of pre-eclampsia, which threatens the life of a baby in the womb when it is rejected by the mother’s immune system.
But the new test picks up an earlier and more subtle red flag in the body – fragments of kidney cells in the urine.
Researchers say these show up well before proteins or high blood pressure, at 27 weeks of pregnancy, so that a woman can be taken into hospital and closely monitored immediately.
The study’s lead author, Dr Vesna Garovic, from Mayo Clinic in Minnesota, said: ‘This important test can tell a woman if she has pre-eclampsia or not within just two hours.
‘Even though early delivery of the baby is the only current treatment for pre-eclampsia, an early diagnosis raises a flag so that a woman and baby can be monitored for their risk of complications.
‘It is a more sensitive test than those which are currently available, and should be available routinely within two years.’
Pre-eclampsia occurs when a mother’s white blood cells fail to adapt to her baby, which is treated as a foreign body.
Around 700 babies every year die in Britain as a result, either through complications of early delivery or being starved of nutrients and oxygen in the womb.
Women, most at risk of pre-eclampsia if they are overweight or over 40, can also die.
Doctors have historically tested women’s urine for proteins, and taken their blood pressure, to diagnose the condition. But previous US research has found fragments of kidney cells appear less than seven months into a pregnancy.
Dr Garovic said these can be detected eight to 10 weeks earlier than the usual symptoms doctors check for.
Indeed, women who develop pre-eclampsia might never develop high blood pressure or urine proteins, which makes the new test far more accurate.
While previous versions have been time-consuming and expensive, the researchers say theirs takes just two rather than 24 hours and is several times cheaper.
The findings, in a study of 84 pregnant women, of which half had pre-eclampsia, are published in the Journal of the American Society of Nephrology.
They also show a blood test can be used to diagnose pre-eclampsia, which in addition to kidney damage appears to cause a surge of foetal haemoglobin – the main oxygen transport protein in an unborn baby.
It is hoped the findings showing the changes in the body caused by pre-eclampsia will provide new treatments for the condition.
Marcus Green, chief executive of charity Action on Pre-Eclampsia, said: ‘Every six minutes a woman dies somewhere in the world from pre-eclampsia – a cruel and terrible condition which also kills 700 babies each year in the UK.
‘The urine test for proteins and blood pressure monitoring are vital and will remain so, but the more we can find out about earlier testing the better for the health of women and their babies.’