A revamped drug that can withstand extreme heat and stay effective for 1,000 days could "revolutionise the ability" to keep new mothers alive, the World Health Organization (WHO) says.
The medicine - known as heat-stable carbetocin - helps prevent sometimes fatal bleeding after women give birth.
It could potentially be given to millions of mothers worldwide.
Existing drugs can help but hot, humid conditions in many countries may stop the lifesaving medicines working well.
About 70,000 women die each year due to severe bleeding after childbirth. These so-called postpartum haemorrhages also increase the risk of their babies dying in the first month of life.
To tackle the problem the WHO currently suggests an injection of oxytocin be offered to all women giving birth vaginally.
But it is recommended that the medicine is kept between two and eight degrees from the moment it is manufactured to the moment it is used, which WHO experts say is often unachievable in countries with little access to refrigeration and unreliable power supplies .
'Very good news'
In the trial, published in the New England Journal of Medicine, almost 30,000 women in 10 countries across the globe were randomly allocated an injection of heat-stable carbetocin or oxytocin immediately after they gave birth.
Researchers found that in most cases, both drugs were equally good at preventing excessive bleeding after birth.
Experts say this paves the way for the heat-stable carbetocin to be offered to all women giving birth vaginally across as many as 90 countries around the world.
And they say it could be particularly useful in areas where the oxytocin is substandard or has been tampered with so there is less of the active ingredient than is desirable.
WHO expert Dr Metin Gulmezoglu said the development of a drug that remained in working order in hot and humid conditions was "very good news for the millions of women who give birth in parts of the world without access to reliable refrigeration".
He added: "It will help to save many lives of mothers in lower income countries where most deaths occur."
And the WHO's director general Dr Tedros Adhanom Ghebreyesus said: "This is a truly encouraging new development that can revolutionise our ability to keep mothers and babies alive."
The drug is already available for other uses and regulators will now look at whether the drug can be approved for wider use.
WHO experts say they hope it will be available in some countries as early next year.
The trial is a collaboration between the WHO, MSD for Mothers and Ferring Pharmaceuticals.